I have a vivoactive 4 connected to an Android phone -- and a bluetooth headset connected to the same phone.
When I receive a call and reject it on the headset, the "call" message does not disappear on the watch. I can tap on the message and see the normal "+49511... is calling", but neither rejecting the former call nor "getting" it would delete the message. Only if I (hours later) call back the same number or get another call, the message disappears.
BTW, don't know if this matters: it is a BT headset connected to two devices at once, and the rejecting happens automatically, when a call comes and I speak on the other device (via Teams etc.). Its a Jabra Evolve2 65.
Greetings, Joerg