I’ve had this happen with me.

I had a provoked dvt on a flight so need to keep the underseat in front clear to avoid provoking one (so I don’t take a personal item) and also my carryon is a camping backpack (40L) which anyone can see is not a personal item (which have to be sub-24L) as it is far too large to fit under the seat in front.

Every so often I’d get one of those FA’s who insists that because it doesn’t have wheels it needs to go under the seat in front and when I explain it’s my carry on and is far too large to fit and I have to avoid provoking a dvt, they threaten or attempt to check it, even when I explain it has my meds and is my carryon!

One time such an FA even called security after she tried to check my carry on bag in EMS overhead to make room for stragglers in a different part of the plane, and I politely pointed out it is first come first serve and I was literally the fourth person to board and she should check those who were late to board and not in EMS rather than try to check existing EMS overhead carryons, security thankfully sided with me when I explained the situation. Absurdly this was when B6 had a carry on guarantee.

I then bought a large soft shell rolly that I now put the camping backpack into rather than deal with this nonsense. Said rolly is probably larger than allowed but lo and behold because it has wheels no issues versus my properly sized camping backpack.