Hey, @jeffchen,
The settings for throughout Zoom can be a bit complex! First of all, I’ll point you to this Zoom Support article which you may have already been through – but just in case someone comes along to view this and hasn’t read the article:
https://support.zoom.us/hc/en-us/articles/115000332726-Using-Waiting-Room
Next, you mention having set the Waiting Room switch on the Settings page, like this:

This option does not require that Waiting Room be set for all meetings. Instead, it sets Waiting Room to On as the default when scheduling a meeting, like this:

You can unset the Waiting Room option when scheduling a meeting, and you can edit a meeting to turn this setting off if it was previously on. Also note that any recurring meetings that were previously scheduled are not affected by changing the Settings-level switch.
I recommend you take some time and go through all of your existing meetings – including your PMI – and check the Waiting Room setting in each individual meeting.
Also note that you can disable the Waiting Room setting in the Security icon once the meeting has started – but sounds like you already know about that. Still, it’s easy to check now and then just to reassure yourself that it is set properly.