ffmpeg cut, encode MKV to MP4, and burn subtitles

I want to cut a section of an MKV and encode it to MP4, while also burning the subtitles.
But I don't know how to do that all in 1 pass (related question), so I tried it step by step.

When I first cut the MKV, and then encode it to MP4, it won't hardcode the subtitles. The cut MKV container still retained the subtitles. I tried both internal and external subtitles.
- test 1 (fail): subtitles directly from MKV
  ffmpeg -i konosuba.mkv -ss 180 -t 30 test.mkv
  ffmpeg -i test.mkv -vf subtitles=test.mkv test2.mp4
- test 2 (fail): external subtitles
  ffmpeg -i konosuba.mkv -ss 180 -t 30 test.mkv
  ffmpeg -i test.mkv -map 0:s:0 subs.ass
  ffmpeg -i test.mkv -vf subtitles=subs.ass test2.mp4
It succesfully hardcodes the subtitles when I first encode the whole MKV to MP4, afterwards I can cut it. But this takes much longer with big source files :(
- test 3 (success)
  ffmpeg -i konosuba.mkv -vf subtitles=konosuba.mkv test.mp4
  ffmpeg -i test.mp4 -ss 180 -t 30 test2.mp4

There seems to be a difference in the logs: test 1 vs test 3

Am I doing something wrong here? I really don't understand why I can't burn subtitles, unless when encoding the original MKV container without cutting it.

Edit: Just trying to burn subtitles by itself doesn't seem to work for me (log)
ffmpeg -i test2.mp4 -vf subtitles=subs.ass test3.mp4

Log: https://gist.github.com/anonymous/d2eb6f09fabba88afe86bd4607b048a4
Build: https://ffmpeg.zeranoe.com/builds/
OS: Windows 10 v1607

https://trac.ffmpeg.org/wiki/HowToBurnSubtitlesIntoVideo
https://ffmpeg.org/ffmpeg-filters.html#subtitles-1