Thralls seem to be getting the same perk really often. Since the Siptah updated I have noticed that thralls get the perk Perseverance more often than any other perk at level 10. Before that it was more randomize but now it seems like 8 out 10 thralls get this perk. Also fighters are still getting accuracy perks.
Please provide a step-by-step process of how the bug can be reproduced. The more details you provide us with the easier it will be for us to find and fix the bug:
1.Level thrall to 10
2.Get survival perk Perseverance