Oracle 12.2 logins hang

we recently upgraded one of our databases from 12.1 to 12.2 on centos 7. we have an application that logs on and logs off for every database operation regardless of size. that results in an obscenely high connection/disconnection rate which eventually (after several minutes) results in all logins to the application account hanging indefinitely on
"library cache lock". there is no lock holder. the session is attempting
to update last logon column in sys.user$ (one of the spare columns).

we are working to address the application issue properly but have implemented database resident connection pooling to temporarily alleviate the issue. we thought that had the issue licked but it happened again on Friday when connection pool was in place. we think there were several sessions created outside of the pool by this account earlier in the day or the day before but i can't see how that would contribute to the problem.