Heartbeat 404

What would cause one heartbeat to return HTTP 200 and another to return 404? (different UIs, same session)

Excerpts from the server access log:
x.x.x.x - - [05/Sep/2023:12:41:54 -0500] “POST /NWD/?v-r=heartbeat&v-uiId=0 HTTP/1.1” 200 - 35719844DDAA2E6BAD691F9A2A89C448
x.x.x.x - - [05/Sep/2023:12:41:54 -0500] “POST /NWD/?v-r=heartbeat&v-uiId=1 HTTP/1.1” 404 716 35719844DDAA2E6BAD691F9A2A89C448

Session timeout / user not authorized anymore → 404 is returned

OK, that makes sense. Just wanted to verify there wasn’t a deeper error condition.