I've never heard of this happening, although maybe it does and we just rarely get odd stuff in the CI logs. But anyway...

AbstractModelControllerClient wraps the ActiveOperation AsyncFuture in another future that under the covers sends a CancelAsyncRequest if the wrapper future's cancel method is called. But it doesn't check the wrapped future's status before doing so or when processing any exception thrown by the call to send the CancelAsyncRequest. Sending the request or propagating any failure are only necessary if the wrapped future is still in WAITING status. If it's DONE/FAILED/CANCELLED just move on and let the calling code report to the user whether the cancel happened.

I noticed this when working on WFCORE-7273 and will fix it as part of work on that issue.