Unable to run K9 (6.400) in windowed mode on Android 12

I have a Lenovo P-12 Pro tablet with Android upgraded to v. 12 by the manufacturer last summer. Until K9 was updated to 6.400, I could run it in a less-than-full screen window just fine. Now it insists on taking up the entire display. The app title-bar displays the icon for windowed mode, but it is greyed-out. I reset the device to as-delivered by the factory and reinstalled K9, but it still insists on full screen.

Am I missing something obvious?Probably a minor irritant to many, but to me…

Thanks in advance!