Here is a result with 21 restos out of 56 rewards:
https://forums.elderscrollsonline.com/en/discussion/324375/maelstrom-post-homestead-50-chests/p1
There's unfortunately no reliable way to infer the statistical odds from such small samples.
I would refrain from making claims ZOS's tables are wrong on the basis of 20 draws, quite frankly.
I feel for you, however, as I'm farming the arena myself
Good luck!