After a forced reboot (and another reboot for good measure), the applications I allow to "Accessibility" control under System Preferences > Security & Privacy > Privacy no longer includes Rectangle.app nor allows me to add it manually.
Helpful but frustrating. What reboots could not fix, a shutdown and start: did.– mcintMar 18 at 7:01
Fortunately I've solve my issue. The solution is frustrating.
What reboots could not fix, a shutdown and start did. Know that system behavior after a reboot may differ from system behavior after a shutdown and start up, manually sequenced.
In evidence, here are some of the Accessibility authorized apps after restart, including the Rectangle app of concern.