I, after achieving mastery rank did not receive the steam achievement for it. This was some time ago so I couldn't manage to replicate it.
So I was playing as usual, then my mastery rank up was available. I then went to do it for the rank of disciple or also known as mastery rank 7.
But upon completion I did not receive the achievement, and still haven't even though I'm MR 16.
I also saw someone else experiencing this type of issue, but on Xbox.
I don't recall what build I was on or what settings I had, since it was some time since I got this bug. I just noticed that I was missing it a couple days prior to writing this report.
But I still haven't gotten my achievement after about a year or so, and I've even tried to replay the mastery test but still nothing.
The images shown above should indicate that I have the achievement but still not on steam.
(could be a steam issue based on the images alone)
Hope this helps with understanding the issue.
I can also link my steam profile if that is needed.