In Premiere Pre 23.1.0 for Mac, trim flags placed in filler will disappear if after a trim.
Happens both in on-the-fly trim mode (setting trim flags on the timeleine outside of official trim mode) and in trim mode (when, say using the +1, -1, +5, -5 buttons)
Makes for a number of problems, particularly on asymmetric trims. If you put a yellow trim flag on on B-Side (i.e. in filler) at tails of a clip on a video track, and A-Side of a a number of audio tracks (not in filler), on first frame of trim (let's say -1), that asymmetric trim will function as expected (Picture will move back 1 frame, audio forward 1 frame), BUT the flag on the video track will have disappeared (images below)
As a result (by example), subsequent -1 trims will move audio back one frame (undoing the forward trim above), and originally-intended trim on the video track becomes an implied (guessed-at-by-Premiere) trim on the video track, which is likely to be at heads on a video clip, where your intention was to handle tails of that very same clip.