Some Results May Not Appear in iOS 18 Mail

Ever since upgrading to iOS 18, I’m getting very limited search results (1 or 2 recent emails) - and the following error in the iPhone mail app:

“Some Results May Not Appear

Mail downloads and organizes messages when iPhone is locked, charging, and connected to Wi-Fi. Large accounts like this may take longer.”

My mail app archive is sizable but it’s been a couple weeks since upgrading - and the message still appears.

I’ve seen a few similar threads for earlier iOS versions but nothing for iOS 18 nor this specific wording.