Is there any way to configure the file conversion to take place when the student uploads it instead of it taking place when opening the assignment? The issue is the same both when using Unoconv or Google Drive converter.
Environment: Moodle 3.7.2, Ubuntu 18.04
Is it possible that your task, Prepare submissions for annotation, is not running? You can check at, Site administration > Server > Tasks > Scheduled tasks and you can tell it to, Run now. It should be running on each quarter hour mark, (15, 30, 45, 00) so if the the teacher goes to grade before the next quarter hour mark passes after submission, they will have to wait. If a quarter of an hour mark has passed since the file was uploaded, it should be ready for marking right away.