Possible Fix
It looks like there's an issue with the "Copy From My Online Assignment Template" option within the Gradebook that prevents templates from showing currently. I've created PR 4950692 to correct this issue and have it show Online Assignment Templates that are created to be listed in that area. I do not have a timetable on when this will be implemented/fixed currently.
As a workaround, have your teacher use the option of "Clone Online Assignment From Other Teacher". This should show the teacher's own classes as well and then you can use the School Year dropdown at the top to choose past Online Assignment Templates that have been created.