Have you started the MongoDB process with version 5.0? You can verify with "db.version() and see if it’s running as v5.0 or 4.4.
The FCV doesn’t upgrade MongoDB, it just enables features for a certain release and helps with backwards compatibility. You don’t want to change the FCV until all the nodes are upgraded to 5.0.
Follow the steps here: https://www.mongodb.com/docs/v5.3/release-notes/5.0-upgrade-replica-set/#upgrade-process