I could have troubleshooted or even just reported this better - apologies and thanks for your effort. Happily the issue has now been resolved.
On more thorough investigation I isolated the issue to >128k M4a/AAC files only. After trying the same files and hotkeys on Foobar with no problem I figured the decoder was to blame as Foobar uses its own.
After a websearch I installed the BASS AAC dll per the FAQ and it works fine now. Sorry again to have wasted your time.
I listen to recorded long radio and pod files where I have L-R hotkeys set to skip 5 seconds forward and back. I'm finding sometimes (but not always) when I hold the key for a longer duration of seeking the player will abruptly switch to the next/previous file in the playlist. Sometimes it occurs on a 2-second hold too though. Per similar types of bug reports I turned off the auto fade feature and that did definitely seem to improve - but not entirely eradicate - the issue.
Use case is that I often play a lot of longer files (radios, mixes, podcasts) where I tend to interact with the progress bar more often than global keyboard controls or controls minimised to taskbar would allow. At the same time however I'd like the player to be as discreet as possible while always visible.