Ok. To everyone experiencing this issue. The likely solution is a full wipe and restore from iCloud or another backup (iTunes if you still use that).
I tested this on my own devices and it has thus far resolved it for stereo recordings. Mono recordings were not impacted by the issue.
This process should be done as a matter of routine when updating major revisions of the OS. And a settings wipe will not sufficiently cleanse rogue or legacy and conflicting settings. Only a full wipe will do it.
Just FYI I’ve been a registered Apple Developer since the program began for iOS and this process has been a necessary evil for nearly every OS release, especially when moving from beta to golden master.
Now that many of you participate in the public beta tests this should absolutely be something you do at the conclusion of a beta. And with major OS revisions like going from 13 to 14 beta 1 etc. you should do it right at the beginning before testing (I.e. a full wipe after installing a beta profile and downloading beta and updating the OS.)
Sadly it’s not ideal to have to do this but it’s been a fundamental necessity for beta testers from the beginning and now that updates are released so much more frequently is highly recommended for users who update through the regular release cycle.
NOTE: Remember to ensure you have a recent backup before doing this to ensure you do not lose any crucial data.