I did some troubleshooting and believe I've confirmed what my initial web searches had indicated might be the case.
It would appear that because the source file is a .mkv with the subtitle stream in pgs format there isn't a direct copy or conversion capability to mp4 container.
I was able to use the custom Encoder type to play with the commands used in the MP4 (Nvidia HVEC H265) and the Matroska (HVEC H265) profiles.
For subtitle copy support the MP4 encoder adds a -c:s mov_text command and the Matroska profile doesn't seem to currently do anything with / support subtitle stream.
So I created a custom Encoder entry merging the values from these two and a ffmpeg doc referencing the copy command for -c:s. Resulting in
Code:
-threads 4 -qp 24 -acodec copy -c:s copy -f matroska -c:v hevc_nvenc -preset medium -y -ignore_unknown -hide_banner
This seems to correctly create a mkv with a copy of the subtitle stream from the original and uses the nvidia encoder to re-encode the video stream.
So questions/ requests.
Is there a reason there isn't a Matroska (Nvidia HEVC H265) that includes the -c:s copy command? Is it easy enough to feature request the addition?
Shouldn't the treatment of subtitle streams be a configurable item in the encoder profile settings like how audio options are handled?
For the conversion error when pgs subtitles are found, I would think ideally the error would be caught and a recommendation made when converting a mkv to mp4 (especially since the canned profiles can't currently have their subtitle options changed). For example if I don't care about the subtitles in the source mkv, seems like I should be able to select and option to ignore the subtitle stream when re-encoding and saving to a mp4 container. Rather than have it just error.
The internet would also make it seem like there are ways to either export for side car use and or convert subtitle stream types to different compatible versions for the different container types. So perhaps longer term feature request would be to support these subtitle conversions within the app as part of the convert process?