Searching for "ContentType:Invoice" gives no results?!?

Hi, does someone have an idea why SP Online search have stopped showing results for "ContentType:Invoice"? I have implemented a solution that relies on SP Search and should display all documents that have ContentType equal to 'Invoice'. It worked fine until one day...

Tried out different ways for the query:

• contenttype:Invoice
• ContentType:Invoice
• SPContentType:Invoice

Have tested the "ContentType:Invoice" on one folder only, but still no results.

I have tested the same search on another tenant where it works fine. Could there be some issue with search service in this particular site or tenant?

Any ideas are welcome...