Mail from some Google addresses not coming through on account in MacOS Mail App

I have an domain/email through IONOS.

Almost all emails come through every hour to all 3 of my Apple devices (Mac Studio desktop, iPad, iPhone.

However, certain emails never show up in those devices' Inbox, nor in the junk/spam folder

The most notable exception is my wife, whose email ends in gmail.com.

I do not have her email (or any other) blocked through my Google account, and many other emails from gmail senders come through fine.

Curiously, if I go into the webmail.ionos.com version of my email account, every email she sent me is right there, in the webmail Inbox, with the proper time/date/subject etc.

So it's obvious that there's no problem with her emails getting through the IONOS server and coming into my webmail inbox. They just don't show up in the Mac Mail client in the Inbox on any of those devices. Yet, all other emails to my address DO show up in the Inbox. I can see 20 new ones from various senders that dropped in the last 3 hours (including others with the gmail.com suffix,) so... Why not hers?

[Re-Titled by Moderator]