HanDuet said:

When deho sent me the updated German subtitles, he also mentioned the following:

deho said:

By the way, I've muxed [the subtitles] with mkvmerge and the resulting mkv played just fine and the subtitles were ok. Then I tried to mux it with tsmuxer, to create a Blu-ray folder structure, but vlc wouldn't display the subtitles correctly, no matter which edit I use. Are you aware of this problem?

Is that an issue someone here could look into? Thanks everyone!

IF the subtitles are in BD-SUP format (pre-rendered graphics, not text), AND the problem he saw was that the subtitles flashed by for a single frame and then disappeared, then yes I've seen this problem. The solution is easy--just import and re-export the BD-SUP file in BDSupEdit. Whatever software originally created the BD-SUP files created buggy (but not atypical) output.

If the above doesn't exactly describe the problem, more info is most certainly needed.