RobG,
I hope this response will help you and any otherswho are on this forum and are dealing with ZEB plushness issues. Your question doesn't detail how many hours you have on the fork? I'm going to go out on a limb and assume you have some hours of riding on the fork. My response below will detail restoring plushness in a ZEB fork that has been ridden for a while and suddenly begins to feel harsh and less plush.
I also have a 2022 Rockshox Zeb Ultimate, 170mm, with buttercups. I've put a little over 3,000 miles on the fork in the last 16 months.
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The Zeb Ultimate is a really good fork, however to be good, it requires frequent maintenance. To maintain plushness, the ZEB requires a wiper seal service every 50 hours of riding. Maintaining the wiper seals on the ZEB significantly impact the fork's plushness. I suspect that this is the reason why Rockshox requires the service every 50 hours and not 120 hours which Fox requires.
When you consider the 50 hour incremant, it really isn't all that much. If you were to ride three days a week with each ride averaging two hours, you'd hit the 50 hour mark in a short time.
You know how the ZEB when new feels super plush???? However, a few months later and for reasons unknown, the fork doesn't feel plush anymore, your fork feels overly harsh and your hands begin to hurt. THe loss of plushness is caused by "Stiction" in the upper stancions. The wiper seals are dry and grabbing onto the upper fork stancions as they oscilate up and down. This in turn causes a harsh fork feeling and loss of plashness when encountering rough trail. When your hands begin to hurt and your fork feels harsh it's the ZEB's way of telling you that the your fork needs a wiper seal service.
The wiper seal on a Rockshox ZEB consists of the rubber wiper seal. The rubber wiper seal prevents trail dirt from migrating down into the fork lowers and eventually destroy the bushings in your fork lowers. In order for the wiper seal to function properly, it requires a secondary foam ring which is situated directly below the rubber wiper seal. The foam ring lubricates the stancion and wiper seal and prevents stiction from occurring. The foam ring also absorbs any dust particles that make it past the wiper seal. If the foam ring runs dry of lubrication, the rubber wiper seal is no longer lubricated and it will begin grabbinig and latching on to your upper fork stancions, thus causing stiction and harshness.
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When a 50 hour basic fork service is performed, a small amount of oil is inserted into each of the fork lower tubes. The fork lower oil serves two purposes, the oil lubricates the fork bushings and also the wiper seal foam ring. Note, all forks whether Fox or Rockshox have this same design.
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Even the best wiper seal is not one hundred percent effective in removing all dirt from a fork stancion as it oscilates up and down. Some dust will eventually make it's way past the rubber wiper seal. Therefore, the foam ring beneath the wiper seal, serves as a secondary defense against dirt making its way into and contaminating the lower fork oil. The downside to all of this is that the foam ring will begin to accumulate dirt and become encrusted with grime. The foam ring will loose its oil soaking sponge like properties and no longer lubricate the rubber wiper seal and stancions.
In extreme cases where a fork is not properly maintained and the foam ring becomes severely encrusted, it will begin scouring the upper fork stancions like sandpaper.
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IMPORTANT NOTE: I've found that to maintain ZEB plushness, one of the most important things you can do is to turn your bike upside down. Leave it that way overnight if possible. You want to do this on a routine basis. By turning your bike over, you are allowing the lower fork oil to flow down and re-hydrate the wiper seal foam ring with oil. It takes a while for the oil to flow the fork bushings and soak the foam ring. Note that turning your bike over for only a few mintures does not allow sufficient time for the oil to work its way into the foam rings. You want the rings to be totally saturated, therefore give the oil time to work its way into the foam rings.
Fox requires that at 120 hours, the wiper seal and the foam ring be replaced. Rockshox with it's 50 hour service, requires only that the wiper seal and foam ring to be cleaned. At 200 hours Rockshox requires that the wiperseal and foam ring be replaced. As for me, I like the silky plushness of a new wiper seal service. Everytime I service my ZEB I replace the wiper seal and foam ring. If money is an issue, you can get by with just cleaning the wiper seal and foam ring.
Performing a 50 hour lower service on the ZEB is not difficult and can be easily done by anyone with a hint of mechanical ability. Performing a wiper service as a first timer will take you an hour or so to complete the process. Once you've done it a few times, it can be done in under 15 minutes. While it's nice to have specialized tools, you can mostly get by without them. I would suggest purchasing a wiper seal installer/driver. They are fairly cheap and will make installing the wiper seal easier and there is less chance of damaging the wiper seal during installation. You will also need the recommended oil. Rockshox wiper seal kits can be costly, i.e. $30. As stated, Rockshox requires that for the 50 hour service, the wiper seal and foam ring need only be cleaned and relubricated. Personally....if I'm going to go throught the trouble of servicing the lower, I'll replace the wiper seal and foam ring.
There are numerous videos on YouTube which detail how to perform a 50 hour Rockshox lower fork service.
Here one such video:
“hmm, I haven’t blown enough money on tweaking my fork yet, wonder if this would work…”. I love the Runt but always wonder if it could be just that little bit more plush. 
