I submitted a ticket on this issue and this was the response from a GM:
Hey there,
Game Master XXXXXX here, thank you for taking the time to contact us I hope this message finds you well.
After taking a look into this, I’ve checked out the logs here and for your followers that aren’t level 100 and Epic quality they’re receiving experience. The ones that are maxed out won’t receive any further experience
Thank you for your time. Should you need further assistance with anything else please let us know. Otherwise, have a great rest of your week.
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Regards,
Blizzard Support
This character has multiple garrison followers since patch 8.3 that go out on missions with other followers and get zero xp. The missions are successful. The ONLY time they receive xp is if they go on a mission by themselves. Something is wrong because that GM is saying his logs are showing my followers are receiving xp. I have looked at the before and after xp and it does not change on those multiple follower missions and the followers are below lvl 100.