Darn it. I just figured it out. Apparently, the custom view was set to be grouped by a virtualtag I made of <Albumartist> (see below) long ago. For whatever reason, the custom sorting rule worked out fine all this time until the latest build. Swapping <Albumartist> out for the virtualtag in my custom sorting rules restored the expected behavior.
Virtualtag (AAPresentation): $If($First(<Album Artist>)=<Album Artist>,<Album Artist>,$If(<Album Artist>="Kairon; IRSE!",<Album Artist>,$Replace($Replace(<Album Artist>*,"; "$RSplit(<Album Artist>,;,1)*,),;,",")" & "$RSplit(<Album Artist>,;,1)))
Sorting Rule (old): Album Artist > $If(<Album Artist>="Various Artists",$Sort(<Album>),<Year>.$IsNull(<DateCustom>,"00.00",<DateCustom>)) > Album
Sorting Rule (fixed): AAPresentation > $If(<Album Artist>="Various Artists",$Sort(<Album>),<Year>.$IsNull(<DateCustom>,"00.00",<DateCustom>)) > Album