Note - I once thought this was a Firestorm Viewer problem, so I downloaded SL viewer per their request guidelines. It has happened with SL viewer too.
At times, I don't receive an IM from someone, so they call me out in local. I try to IM them to correct it - and I question if they can see my IM. I see that they are responding, but then there is no message. I have finally caught this whole transaction in screen shots. I don't have trouble with local at all. I have tried to re-log and it sometimes still does not correct the issue. The issue is also very random. Some days it does it and some days it does not. I have noticed it on two different PC's too - both running up-to-date Windows 11. I just recently downloaded the newest version of Firestorm release, hoping it would clear up. But it just did it tonight (11.15.25)