To answer myself: there is a known [3.4.0.22] Screen Rotation doesn't work in Android Apps issue which also refer a https://talk.maemo.org/showthread.php?t=101015 thread with more details. Sorry, didn’t find this before…
The suggestion (vertical/dynamic display orientation, start/stop AlienDalvik) from the thread didn’t work initially but suddenly later after I run the sailfish browser in vertical position and start/stopped Dalvik some more, Brave rotated to vertical. Strange. It’s also strange that it worked here (XA2 with 3.4) when the problem resurfaced for the maemo thread author with 3.4 (on XA2Ultra).
(I mark this as solution (explanation) and - if possible - will close here)