SUMMARY
This one is weird. Since updating to the latest TW snapshot with Plasma 5.21.3 and Frameworks 5.80.0, Dolphin seems to read-lock the *first* file being played from an NFS mount by mpv.

STEPS TO REPRODUCE
1. mount NFS share
2. play a file with mpv
3. close mpv

OBSERVED RESULT
Read lock on the file by "file.so" persists. This means you can't delete the file or the folder containing it.
Further files played with mpv lose their read lock some time after closing mpv.

EXPECTED RESULT
Read lock removed

SOFTWARE/OS VERSIONS
Operating System: openSUSE Tumbleweed 20210319
KDE Plasma Version: 5.21.3
KDE Frameworks Version: 5.80.0
Qt Version: 5.15.2
Dolphin 20.12.3
Kernel Version: 5.11.6-1-default
OS Type: 64-bit
Graphics Platform: X11

ADDITIONAL INFORMATION
- playing a file with VLC does not spawn a "file.so" thing.
- the following OS/KDE versions do *not* have this problem, and don't spawn a "file.so" from Dolphin either when playing a file with mpv:
Operating System: openSUSE Tumbleweed 20210307
KDE Plasma Version: 5.21.1
KDE Frameworks Version: 5.79.0
Qt Version: 5.15.2
Dolphin 20.12.2
Kernel Version: 5.11.2-1-default
OS Type: 64-bit
Graphics Platform: X11


Some command outputs of the *affected machine*:

# lsof /nfs
COMMAND  PID USER   FD   TYPE DEVICE SIZE/OFF      NODE NAME
file.so 4389 user    5r   REG   0,99 43362444 138809846 /nfs/video.mkv (10.0.0.3:/export)


# ps aux|grep 4389
user      4389  0.0  0.0  87696 19244 ?        S    18:13   0:00 file.so [kdeinit5] file local:/run/user/1000/klauncherQcszyt.1.slave-socket local:/run/user/1000/dolphinTTHkjd.9.slave-socket

(it's still lingering 20 minutes later)