Stop iPhone from adding events (Exchange)

I have an Exchange account for work linked to my iPhone. Based on my work policies, I get forwarded invitations when someone books certain conference rooms. These are actual invitations, but they are really only on the conference room calendar. In Outlook, they never show up on my personal calendar. Also, I have a rule that sends them to a specific subfolder, so they don't even show up in my Inbox.

What this means is that I never see them on my phone (because I only ever check my Inbox). And they are never on my actual calendar. Both of these things are exactly what I want.

The problem is that they recently started showing up on my iPhone calendar. I cannot figure out why. The phone never sees them in e-mail, and it can't be getting them from my calendar. I tried changing my Outlook rule to just delete them instead of moving them to a subfolder, but that didn't change the iPhone behavior. If I go to Settings and turn off Calendar for this account, then turn it back on, they're all gone. This makes sense since they're not on my real calendar. But the next time someone sends one, it goes on the iPhone calendar.

How can I stop my iPhone from doing this? I've turned off Siri for scanning e-mail and calendar, to no avail.

Is there a way to tell the iPhone never to get appointments from anything but the calendar?