I’ve been having this same issue for at least the past several months and it’s getting pretty annoying since I set timers for myself for work-related tasks.
I will set a timer for X minutes and, I’d say half the time, the alert doesn’t go off until I raise my wrist. I’ve sometimes looked down at my watch while lowered, seen the screen sit there with it showing 00:00 remaining, I’ll raise my wrist and then the timer alert goes off.
I have an Apple Watch 5 with the latest update. I unpaired the watch, and set it up again as new and still have this issue.