strange behavior of mpv player

[osmium]

I use the following command to play multiple videos with mpv and compare them side by side:

mpv "D:\input.ts" --external-file="D:\output.remuxed.ts" --lavfi-complex="[vid1]scale=600x400[v1];[vid2]scale=600x400[v2];[v1][v2]hstack=inputs=2[vo];[aid1][aid2]amix=inputs=2[ao]"

When playing 2 copies of the same file they play in sync as expected and both audio and video remain syncronized during seeking.

If I remux one of the files:

ffmpeg.exe -i "D:\input.ts" -c copy "D:\output.remuxed.ts"

and play them together they start playing in sync, but when seeking the audio tracks run out of sync with 1-2 seconds delay between them (video streams are fine). I find it strange because all the streams preserve their timestamps, start times etc. during the remuxing process.

Is there something wrong with my remuxed file or it is just some issue with mpv player or the command I'm using ?