Searching Mails on the iPhone confuse the dates

I have seen that in the last time (about 6-9 months) if I search on my iPhone in the Mail.app the results are not logical listed. Sometime begins the output with the newer messages, but than comes up messages that are many years old.

Is this a bug or must I reset my iPhone or Mail or mac-Account?

Is this issue known and I'm alone with this problem?

Any help will be appreciated

Ciao