I'm on Debian 9.8 on server side, running the latest beta, but this issue is there since I upgraded to Emby 4.0. (was working fine with Emby 3.5.2/3.5.3).
When playing MKV files with ASS Subtitles, tested on Firefox and Chrome (HTML5 playback), subtitles are not showing.
I checked server logs, and found that ASS track is encoded/demuxed via ffmpeg:
./embyserver.txt:1029:2019-03-11 13:22:56.065 Info SubtitleEncoder: ProcessRun 'ffmpeg-subtitle_extract' Execute: /opt/emby-server/bin/ffmpeg -i file:"/srv/media/Series/Sword.Art.Online/S03/SAO.S03E21.VOSTFR.1080p.WEBRiP.x264.mkv" -map 0:2 -an -vn -c:s copy "/var/lib/emby-server/cache/subtitles/b/b3fd429fcf754f26313b86b441d5104e_636877678715979463_2_0_0_False.ass" ./embyserver.txt:1079:2019-03-11 13:23:17.462 Info SubtitleEncoder: ffmpeg subtitle extraction completed for file:"/srv/media/Series/Sword.Art.Online/S03/SAO.S03E21.VOSTFR.1080p.WEBRiP.x264.mkv" to /var/lib/emby-server/cache/subtitles/b/b3fd429fcf754f26313b86b441d5104e_636877678715979463_2_0_0_False.ass
So it seems that's why subtitles are not available, but when the process is finished, Subtitle doesn't show up, I need to disable and enable them back again (and seek back to the beginning of the file).
With big movies, we have to wait many minutes for the ASS to be "available".
Also when streaming (from Android app) to my Chromecast (tested with Chromecast v1, v2 and Ultra), thoses ASS subtitle are not available at the begining and then, they show up, but accelerated, and are kind of out of sync with audio/video. We must stop, disconnect and restart the stream to have them properly displayed. Sometimes they don't show up at all.
Let me know how I can help to debug those ASS playback issue.