I'm using the Magisk version of Xposed. It did have a newer version. Upgrading it is not an experience I'd like to repeat. That and the new installer broke everything. Eventually I had to uninstall all modules and both apps, wipe cache and data and restore all apps and settings from backup.
So now Android Auto works as long as it isn't plugged in to the head unit. Xposed Installer sees Xposed and loads the SAAX module. But Android Auto is back to flashing on and off. In notifications, Maps is repeatedly loading and unloading. The Xposed installer log sees SAAX being loaded every few seconds at the same time that AA is flashing.
As it happens, I have SAAX versions 1.3, 1.5, 1.7.2, 1.7.8, and 1.8.8 stored on my computer. If any of those are ones you are missing, I can send them to you.
Just to verify, you have AA 4.2.591434 or 4.2.591444 installed? I downloaded both APKs again from APKMirror, decompiled, and ran a diff to make sure the code is the same between them.