Next time I used a new Bus, AUX 20 was created again, and again it was unusable in the described way... So I again created a new AUX, routed the Bus to it, then just detached the Bus from AUX 20 and switched AUX 20's output off; this way I am "parking it nowhere" so it won't be used again.
I tried a new project, just one instrument strip, and created 25 AUX objects in the Mixer; they all do their job, including AUX 20, so it's not the number...
MBP 16" i9, Logic Pro X 10.4.8, 4GB, macOS 10.15.3 (Catalina)
Edirol FA-101, FM8 v1.4.1, Absynth 5.3.1, Pianoteq Pro v6.0.1, Reaktor 6.1.1, SWAM Clarinets & Saxophones, Izotope stuff
CME UF80, TEC USB MIDI Breath & Bite Controller 2http://soundcloud.com/joerg-pegler