What i can see is that sometimes a player with stamina20 has an absurd low initial form for the given game, which then elevates to normal level as the game progresses. I have no hard evidence whether it is a display thing or real, but based on some results i suspect it is for real and could account for some losses.
On the basis of rather be safe than sorry, i personally do not touch stamina 20 players. (Also, it is not that hard to avoid them, so the cost of avoidance is low.)

I had a stamina 20 player on my team last season (Libor Svoboda, I played division II.3, you can check out his performance for yourself if you want).
I was fully aware of the bug, even when I bought him, but he performed well enough to keep him.

What happened was that about half the games he started with 5 stars and ended with 5 stars, as you would expect with 20 stamina. The other half of the games he would start with 5 stars and end with 4 stars, as if he would have a stamina of below 10.
This 50/50 behaviour could be related to whether the consistency rng came up with a positive or a negative number. Although he would score the same for both positions I played him on and I was under the impression that if a player plays two positions the consistence dice is rolled for both positions independently.
Anyway, half the time he was an absolute stud, half the time he was merely ok. And that is by looking at his game ratings and I don't know what they are worth. In summary I never saw a reason to get rid of him.
Since many players avoid stamina 20 players like the plague I assume you got yours cheaply, so you might have gotten a great deal...

I've always thought that the impact of the bug depended on which position(s) it involved. Having it occur at QB, RB, FB, WE, TE, or CB is likely worse than at OL, DL, LB, SF. Basically, the large independent role a player has on the game the bigger the impact.