I recently found that searching for anything using the search bar on top next to the SE logo will result in "no matches". For instance, searching for the keyword "Integral" yields:
which is clearly wrong.
This is not an answer, just an attempt to help to clarify that there is (or at least was) actually some issue. (I will delete this later when it is no longer needed, I just wanted to add some screenshots documenting the problem.)
Other than the search taking unusually long, sometimes I got results, but sometimes I got none.
The following two screenshots show what I got when searching on the main https://math.stackexchange.com/search?q=integral - they were taken within minutes of each other.
The same thing happened to me when searching on meta https://math.meta.stackexchange.com/search?q=integral - again the two screenshots were taken relatively close after each other.