Audio out of sync and the desync keeps increasing as the movie continues

Okay, so I had a movie in 1080p in english audio (mp4 format). I was like 1:25:17 in length.

I recently got a 4k TV and wanted to watch the movie in 4K (MKV format). I downloaded it but it wasn’t in english and also, the length was 1:25:25. I watched where the other 8 secs are and its in the end after credits so the audio is the same time length as the movie.

However, I used MKVToolNix to put the 1080p’s audio on the 4k one. It started off as synced and all good. But then it got a little out of sync and the desync kept increasing as the movie went on.

Can anyone help me out here? I played the movie side by side and the audio just gets out of sync idky.