heyhey It is an expected behavior inherited upstream from AOSP.
When you receive a phone call in an app like WhatsApp or Signal from another user, the call UI is shown but you can't pick up or decline the call, nor can you switch user. You have to wait until the ringing has ended, then switch to the other user, and ring them back.
The same does not apply with apps which do carrier-based calls, or with clock apps. You can pick up a carrier-based call or snooze an alarm from other users, which leads me to believe it is something to do with direct boot (have not confirmed this). Apps like WhatsApp and Signal don't get access to direct boot.