I just tried it with

ffmpeg -c:v h264_qsv -i Kimetsu\ no\ Yaiba\ Yuukaku\ Hen\ -\ 01\ \[Blu-Ray\]\[1080p\]\[10bit\]\[BCEFB086\].mkv -map 0 -c copy -c:v hevc_qsv -preset slow -global_quality 22 -look_ahead 1 output.mkv

and got:

[h264_qsv @ 0x5647e6354780] Error initializing the MFX video decoder: invalid handle (-6)
Error while decoding stream #0:0: Invalid argument
[h264_qsv @ 0x5647e6354780] [IMGUTILS @ 0x7fffc3312e40] Picture size 0x0 is invalid
[h264_qsv @ 0x5647e6354780] video_get_buffer: image parameters invalid
[h264_qsv @ 0x5647e6354780] get_buffer() failed
Error while decoding stream #0:0: Invalid argument

Could this mean it is a driver issue?

There is 1 plugin that seems to support H.264, the rest are H.265. I tried the following plugins:

• “Transcode Video Files” with the following options selected
  • Video Code = H264
  • Video Encoder = hvec_qvc
• “Video Encoder H265/HEVC - hevc_vaapi (INTEL/AMD)”
• “Video Encoder H265/HEVC - hevc_qsv (Intel)”

None of them worked.

Is there something similar on fDroid?