Magic keyboard (fully charged) does not connect at the login screen after 15.4 update (2019 iMac)

I have a Magic Keyboard that connected flawlessly to my iMac prior to 15.4.

After 15.4, my Magic Keyboard doesn't connect at the login screen, making it impossible to enter my password.

I CAN login if I find a lightning cable and connect the MK manually.

Again, this MK worked great (and wirelessly) prior to the 15.4 update.

Under 15.4, this MK works wirelessly AFTER I have logged in using a manual wired connection.

iMac version: 2019 27-inch 5K with i9