Yeah I noticed the same and compiled some data on rings and it seems like it’s more like 10% as well.
I got 52 legendaries on 500 gambled rings, the probability of getting 52 or fewer legendaries on 500 attempts, if the true probability were 20%, is about 1 in 178 million.
Many other slots (e.g. sources) appear to be working fine, I get ~20% there.
But it does seem like a few slots may be bugged and still only give 10%.