Keyboard Maestro Can't Type Shift-CMD-F16?

OK, then it is not an accessibility issue.

Seems like it is probably this issue:

Catalina doesn't support triggering hot keys with simulated keys for function keys or arrow keys, but weirdly does allow it for other keys. Whether this is a bug or a “feature” is unknown. If you can change the target hot key to something other than an arrow or function key, then Keyboard Maestro can trigger it based on simulating the key.