Audio track not playing in Multiplex'd title

I’m adding Subs to an anime that has both Japanese and English audio tracks.

The “ja” is set to “default track” (yes) and “track enabled” (yes)
The “en” is set to “default track” (yes) and “track enabled” (no)

The issue is that the tracks copy into the .mkv file but one of the tracks plays silence when selected. Both audio tracks play fine in the .mp4 but only one plays in the .mkv.

Can someone help with this problem? I’ve tried to search for answers but have had no luck so far.

Much thanks for any assistance,
Nic