In the OP you say you tried sqlsni.trace, I pointed out you should try sqlsni.sni_trace (not the same event). The former would have worked on SQL Server 2016 (which is...

You may have found this by now, but the event you want to use is sqlsni.sni_trace for 2019.

https://dba.stackexchange.com/questions/305966/monitoring-sql-server-encryption-with-extended-events

This isn't shown in the list of available events in SSMS, even though...