If I use the search in an imap account the results are not complete.
I opted to search into the mail text: (Textteil)
It seems that it finds old mails but not new ones.
If I search for “Sendungsstatus” I can find DHL mails which are 5 months old. But newer Mails with this string are not listed.
I also activated server search and I also selected that the mails are downloaded for offline use.
A server search has a limit on how many results it returns, so even if there are more messages that match you won’t see them. Can you untick that option and try the search again.