I have a level 266 3* Magneto, and a level 140 2/5/4 duplicate. No saved covers.
I have a level 144 2* Magneto, and a level 126 duplicate.
I got a 2* Magneto cover, clicked Add Level. Level 127, next reward, yellow 3* Magneto cover. Boom.
"Puzzle Quest has stopped. Close app. Send feedback"
Reopening the game, my 2* is still 126, and the cover is still in rewards.
This has happened on both my Samsung S8+ and Nexus 9. I gave up and sold that cover. Then I got another cover, and thought, why not try again? Same problem.
I've applied other covers successfully.
:edit: It happened with 2* Daken too, but other covers (that don't award covers in the next tier) are fine. So far. (Daken would have given a 3* Daken cover.) So I've edited the title.