Lightroom Classic: Add to Target Collection B Key Shortcut Toggle Not Working Correctly (Windows)

In the Catalog panel, is there still a + symbol next to Quick Collection when this is happening?

Yes

Is this consistently reproducible? I am not able to reproduce the issue on my end. 

Could it be possible that you pressed B twice in quick succession and Lr is slow in responding to process the key command?

It's inconsistent. As I indicated, merely restarting Lightroom without a machine reboot can fix the problem. Once present, it persists throughout a session until LR is closed. I have not identified other conditions that must obtain to reproduce the issue. If I do, I will post the specifics. Importantly, I have observed this issue several times since updating. It occurred yesterday and then again today right after the machine was first turned on and LR opened. Being certain to press the B key just  once does not require a piano virtuoso. It's not a press twice issue. When file and collection handling was altered in this update, this new bug was introduced.  Thanks.

Try pause the Lr mobile sync from Ligtroom's top left identity plate window area, and relaunch Lr. And see if it happens again and report back.

Didn't reproduce issue. B key works as expected even after turning off Sync and restarting LR. After using B to add or remove photos, I confirmed that Sync was still set to Pause post LR restart.