I assume you’re using the Play Store’s version of Symfonium?
Possibly related to this post which was resolved for the beta a while ago.
If you’re able to share a link to the TTML file, I can try to test to see if the newest release has fixed this already. (Via Local Media anyway. I don’t have any server stuff set up.) Or you can sideload the newer version yourself to see if this still occurs. If you do, be sure to make a full backup in Symfonium beforehand in case you want to downgrade!
If you’re already using the newest “soft release” version, then the dev will have to look into it.
(Note: I’m just another user, not affiliated with the dev.)