I also got this issue with family members in my own court (3 empires and counting).
Even with courtier or vassals even baron, count or duc.
I usually pause the game as soon as I get the message of a dynasty member becoming decadent (to know if it is my courtier or a vassal barony, count or duc style).
Then I display the character and the decadent trait as already been removed, I double check with the religion tab and see nothing. I don't have any option to negociate or any message from my vassal telling me the courtier, baron, count or duc telling me the person is kafir to emprison them.
=> I read in a CK2 wiki that dynasty members having a title equal and/or over Count title should not become decadent. Alas it happens from time to time.
My work arounds to avoid the decadence trait:
1/ in order to lower the dynasty members to get the trait, I force them to marry 3 or 4 wifes, it seems to lessen decadence trait occurence.
2/ I have granted a Jew a King title, when possible, I ask family members to be educated by Jews in my realm, so they become Jewish and never become decadent at all.
BUT all of these work around are consuming a lot of my time, especialy in the middle of a crusade!
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As stated in a previous post, at the opposite side, I sometimes never get any message from a dynasty member vassal (count, duc, king) telling the familly member is decadent and now kafir allowing me to emprison them at no tyranny penalty cost.
(once, the family member was part of the council of Count (in my dynasty) but the count better leave the decadent member than firing him from the council)
I once had a decadent duc, I have tried to emprison him and failed, he moved at the court of one king in my realm (king not from my dynasty), I was forced to emprison him again getting another penalty for being tyrannic.