I'm not sure what the cause may be, but there is somewhat of a workaround / solution to resolve this bug, at least temporarily.
This has just happened to my editing session and the selection based processing has auto-taken over my bus 1 send, and i only realized it when i went to create my first bus group and saw bus 1 labelled "selection based processing A". Somehow the selection based processing A channel has created bus 1 as it's input source, and is a solo safe channel, hence you having trouble with your soloing issues. To get rid of this, open up your midi environment window (Command 8) and find the 2 channels that have been created (Selection based processing A & Selection based processing B) and delete those channels, OR re-address their input source to something other than bus 1 (for A). I just deleted those 2 extra channels because i wasn't using any buses to begin with, and selection based processing shouldn't hold a bus input anyway is it is a temporary audio suite - like momentary audio editing tool, but i can see it being a little more complicated if i've had a session built up already with specific bussing and routing going on, and i would want to make sure that my bus 1 channel is still the right one as far as the inserts i had allocated to it... at that point i would just select no input on the selection based processing channels within the MIDI Environment window.
Hope that helps!
OS 10.15.7. Mac Mini (2018), 3.2 GHz 6-Core Intel Core i7. Memory: 64 GB 2667 MHz DDR4. Logic Pro 10.6.1. SSL 2+ Interface.