Powefly 5 Keeps jumping out of top gear

[SuperFunk]

Help !!!
My Powerfly 5 2020 has kept jumping out of top gear ( the smallest one ) on a 12 speed cassette, I originally thought ir might be the derailer which is a sram sx as it left the factory. So i swapped it for a shimano one, but whilst it was much smoother in the other 11 gears the last gear it just jumps as if the chain is going to come off. So i then changed the cassette just in case it was that, it looked fine but i changed it anyway. Again still the same problem. It's been to a couple of bike shops and they seem to think it's fine as it works great on the stand but only seems to jump when underload. As i say every other gear is fine.
It's got to the point where i'm seriously considering replacing the bike !
Any help or pointers would be appreciated.

 

[Asnowmobiler]

Mine did that when I mistakenly installed the master link upside down.

 

[Mikerb]

The smallest cog has the least chain wrap. Did you change the chain when you changed the cassette?
Potential causes assuming the derailleur low stop is correctly adjusted
Worn chain on a new cassette
Chain too long
Insufficient rear mech clutch tension
Chain line wrong ( have you changed the chainring?)

 

[SuperFunk]

Sorry for the late reply, But thank you. You were spot on, i'd some how put the chain link on the wrong way round. The anoying thing is that it's been t 3 diffrent bike shops to be checked and none of them picked it up either !!