I have no filters applied. I'm aware of the "unread" icon and its purpose, so that's not the problem. I spent years as a Mac software developer, so it's not an issue of simple ignorance about settings.
For greater clarity, here are some screenshots comparing the same search in different apps
First, the search for M&M (the store is named M&M Food Market) in MacOS 13.1 on a Mac Studio brings up the Email from Monday:

Next, the search for M&M in iOS 16.4 on an iPhone 14 Pro does not bring up this most recent email, just emails from years ago (which are in the trash folder since I don't filter out mailboxes):

Next, the same search in iOS 16.3 on an iPhone 11 Pro which has the same account settings so it's reading the same IMAP server. Here it correctly brings up the most recent email, just like MacOS:

Next, the same search in the Zoho Mail app on the iPhone 14 Pro. Because the Zoho app does searches differently, it brings up a variety of references, but still shows the recent email from M&M Food Market:

Finally, the same search in another iOS app, Edison Mail, brings up different search results. Again it's due to how this app handles searches but once again it correctly shows Monday's email from M&M:

It's pretty clear that something changed in iOS 16.4 that is affecting searches. Either it's a bug in the software, or some change in the settings. If the latter, I'm at a loss as to how such a change would have happened as my iPhone 11 Pro and iPhone 14 Pro are running identical settings, just different versions of the OS.