I am convinced that most if not all the mentor issues come from the mount rewards for 2k mentor roulettes. 2 issues with that:

1. The number of clears required is insanely high - encouraging mentors to go after quantity over quality.
2. Every duty is treated as the same "progress" towards the mount. So a 2 minutes guildhest is worth the same as a 1-2 hours ex primal.

There's ways to solve it but SE has refused to look at the problem. This leads to a lot of mentors abandoning ex primals because they are not worth the effort - even if you end up clearing after an hour (which is optimistic) it is less progress than leaving instantly, eating the 30 mins penalty and possibly ending up in a guildhest.

Some possible solutions:

1. Stop considering number of clears as the unit towards the mount. Instead, count the time spent inside a cleared duty. So if you spend 2 minutes in a guildhest it is worth much less progress than spending 2 hours in an ex primal that you end up clearing.
2. Split the ex primals into a separate mentor roulette, and give it greater rewards. This would ensure only mentors who want to help for this specific content are there, and that they are rewarded in consequence.