Many requests show "running normal" with no actual_start_date

Product/Version: Oracle E-Business Suite 12.2.8

On 3 occasions (over the last year) around 100 processes have shown on fnd_concurrent_requests as "running normal" (phase R status R) but there is no actual_start_date, and they have no active database session. This means the concurrent manager thinks they are running, but they aren't. Some have remained in that state for 6 months (waiting to be terminated). They can't be terminated via the front-end. The cause "incompatibility", preventing other programs from starting (they remain at Pending Standby).

My question is:

Does anyone know why this would happen, and can we avoid it?

I suspect it happened during patching at a weekend - we have Editioning, if that's relevant. Our DBAs "chop" concurrent processes during maintenance periods when the system is restarted, but the above state doesn't usually happen.