My OS Maps used to do that sometimes but not for the past year or so, and I also no longer have to login to view my offline maps on the OS app. Think it was a problem with the desktop site and the mobile app not remembering each other.
Mine still does this, running current OS Maps app on current iOS. I am fairly sure it’s a license check. Don’t think it’s related to the desktop site because I’ve never used that.
Probably this is due to usually only opening the app when I’m lost in the woods and want to know which direction to head. I suspect if I opened it somewhere I had internet access before getting lost in the woods, it would be ok. But I’m seriously thinking about cancelling my subscription and trying an alternative.

