JBrasell Thank you for the information. So far, I could not reproduce your problem on any of my devices. But I've researched the issue, and it could be that this is a system-wide search indexing problem in iOS 18. There is a thread in the Apple discussion forums in which over 300 people say they have similar Spotlight indexing problems.
So it would be interesting for me to know if other searches are affected on your device as well. For example: Can you find a Pages document by searching for contained text?
In the discussion thread, they mention a workaround that solved the problem for some people. It is easy to do, so trying it could also shed more light on the issue. This is what they suggest:
Go to - Settings/Privacy and Security/Location Services/System Services/Suggestions and Search: Turn the switch to off then on. It will take a while (the more stuff you have, the longer it takes), but it will re-index your entire device. Hope I remembered it right and that it helps.
There is also a video showing the procedure.