Although the "Tabbed forms" field had been set to true in the User Preferences (tabbed.forms preference), no users were actually seeing this reflected in the UI.
Instead, they found that the form sections were displayed as if it were set to false. They were also finding that they were being switched to UI15, instead of the expected UI16.
The documentation for User Preferences states the following,
Note: Having more than 10,000 user preferences causes system degradation and UI performance issues.
It was identified that the customer in question had over 78,000 User Preferences, of which 10,074 were of type ‘System’. Of these ‘System’ User Preferences over 9,000 were identified as having been accidentally created and were deemed as being junk.
Deleting these junk ‘System’ User Preference records so that the total count was then well below 10,000 resolved this behavior.