I tested on my Moodle 3.6.1+ (Build: 20181220) with 3 PDF's. All three were converted and I could mark on all three of them. I just upgraded the to Moodle 3.6.2+ (Build: 20190208) and using a student uploaded two PDF's and an odg file. The first student acount still showed the rendered and annotated 3 PDF's. The new student upload rendered all three files correctly, I cound annotate on them, and the student could see the results.
Do you have any restrictions set for Submission types > Accepted file types, in the activity setup?
Under Site administration > Plugins > Document converters > Manage document converters, are the file types you are trying to use, all listed under Supported document conversions column for Unoconv?
Thanks for letting me know. I found the arrow buttons on the top left side to move through the files. Now that I've figured out the basic functionality of this it works. Thanks for testing this and providing your feedback. I was looking in the wrong spot.