Subtitles don't show on Apple TV 3

I'm using a Apple TV for a few years now and I never had this problem before.

I play mp4 files from my macbook pro to Apple TV 3. I mux in the subtitles myself using programs like Subler and MP4Tools from SRT to Tx3g.

But when I play the file on my Apple TV the subtitles don't show. I have selected them in Captions, like always, but nothing. With one file only the first few lines were shown but later disappeared. Some files play without problems.

If I play the files on different media the subtitles show without problems

Does anyone have the same issue or a solution for this problem?