UPDATE. I'm not posting this as an answer since it provides none. When 5.34 became available, I tried an update on this site and the problem went away. I guess there was an issue with 5.33.2. I'll likely never know.
Weird situation. I have just upgraded to CiviCRM 5.33.2 and Stripe 6.5.6 on Wordpress 5.6.
When I test a Contribution after the upgrade, I get an on screen error saying "Failed to update contribution in database".
I reviewed the logs for clues and found nothing very useful. The Apache error log is clean and the Civi log (even with debugging on) reports only
Jan 21 14:59:56 [info] Contribution record updated successfully
Reviewing Stripe directly shows a successful (test) payment and CiviCRM shows the same.
I have tested a single live payment just to see if there is any difference. It behaves the same way as the test payments (as it should).
The only additional clue I am receiving is that no email is received either. However, testing the outbound SMTP settings returns success and the test email is received.
Any thoughts on where to look for clues?