Hi macnpchz,
Thanks for using Apple Support Communities. Perhaps changing your search criteria may help in some instances. For instance, using text from the email or searching for attachments. Some examples can be seen in Search for email in Mail on iPhone. However, since this has been happening for a while and on multiple iPhones, you may have tried those tips before.
Next, make sure that Siri and Search are able to check within Mail. Check out the following guidance from Use Search on your iPhone, iPad, or iPod touch.
"Change search settings
You can change the search settings to limit which apps and results appear while you're searching. Here's how:
- Go to Settings and tap Siri & Search.
- Scroll down and select an app.
Then select Show App in Search, Show Content in Search, Show on Home Screen, Suggest App, or Suggest Notifications."
If the same occurs, try using the Search bar on your Home Screen and type in the same criteria that you would use in Mail. Then, scroll through the results from there. You may be able to find the email that way.
Take care.