VLC can play WebM files but not MP4 while being rendered.
Why can MP4 not be played while the video is being rendered?

I guess there is either some metadata information missing or data referenced in metadata that has not been renderd so far. But: Just a guess.

MOV/MP4 have "faststart" enabled, to allow playing while downloading... this means writing the packets data at the beginning of the file, but full information is available only once the encoding is complete, so you can't play the file while encoding. If you want to do it, you must create a new MP4 profile without faststart (and might re-wrap the file in the end with FFmpeg).
WEBM has a different streaming approach (thought in the format from the beginning) so it just works.

If your VLC is outdated, such errors are quite possible. Update it to the latest version. You can even watch movies with VLC stream feature.
https://neptunofilms.com/free-movie-streaming-sites/