Adding myself to the mix here in a “stand up and be counted” way —Also experiencing this with the following relatively-stock setup:
- Mac mini M1 (macOS 15.3.1) connected to…
- Synology NAS (DSM version
7.2.2-72806) via… - SMB w/ default macOS & DSM configs (nothing tweaked so far in
nsmb.confor on the DSM side) - Connecting using AutoMounter (with the mountpoint mapped into
/Users/Shared/though) - Latest Plex Server release as of this writing (
1.41.4.9463-630c9f557)
Previously I was using an rclone SMB remote mounted using the rclone mount command via FUSE-T. That setup displayed absolutely none of these issues, which makes sense since it uses the go-smb2 library not Apple’s own SMB stack… but I noticed that was only getting about half the expected hardwired throughput performance.
This wouldn’t be an issue for playback (I don’t have any files that exceed 50+ megabytes per second bitrate!) but I recently enabled thumbnails, credits detection, voice detection, etc. across my libraries, so I was looking for the fastest reads possible, which, in some brief testing, the built-in SMB stack seemed to offer.
Buuuut, as we all know, it apparently comes with this distinct downside, where more than half my movie library ended up with 
trash icons on 'em, plus “File is missing” errors on some playback attempts (but not across the board… sometimes a file showing would still play as expected, which wouldn’t actually remove the icon but it was something at least).
Thankfully I’ve never had “Empty trash automatically” enabled so no real harm done to metadata, etc. and for now I am testing AFP via AutoMounter just for funsies (so far: same speedy throughput as SMB w/ no Plex-trashing issues) but I may end up just going back to that rock-solid rclone setup once I get an initial thumbnail-creation pass done on my libraries.
Anyhooo, will be following along for any updates/findings as they develop! 
