The UTXO used in the coinjoin transaction doesn't have any unconfirmed parent at all.
If each of the inputs that are used in the RBF-disabled CoinJoin don't have an RBF-enabled unconfirmed parent, then what's the problem?
That UTXO was combined with another UTXO in a different transaction at the same time as the coinjoin transaction.
The CoinJoin transaction is consisted of some parties' inputs. One of those inputs might be an RBF-enabled unconfirmed parent, which can be abused by an attacker to disrupt mixing. Can't you give a solution to this if you make CoinJoin unavailable for RBF-enabled unconfirmed inputs?
As both transactions have been made at the same time, it's possible that the client see the coinjoin transaction before the other one and doesn't understand what's causing the issue.
I must have messed it up in my mind. Isn't CoinJoin a child of the RBF-enabled unconfirmed parent?