Waiting Room - unable to join after Waiting Room is disabled

We had a meeting in progress.  At one point, others were to join us, so the Waiting Room was turned on.

When the scheduled time for them to join arrived, they were all admitted. Then the Waiting Room was disabled so that late comers could immediately join.

However, someone joining later could not join. (Not sure what message they saw.)

Only when the Waiting Room was enabled again could they join the meeting.

Is this a bug in Zoom?  I would expect that when the Waiting Room is disabled, people should be able to join as they normally would.