The issue (for me at least) appears to be with the Microsoft Co-Editor Executable which is part of OneDrive, the file is called FileCoAuth.exe. I installed the Windows 10 2004 preview to try to fix this issue and when I went to search again in Explorer it was still blocked, but as soon as I tried to click in the search field my firewall notified me that FileCoAuth.exe wanted to access the network and as soon as I allowed it the Explorer search started working. (I have my firewall setup to ask me anytime something wants to access the internet).
So maybe there is a solution to this bug without having to install the 2004 preview. First thing I would try is creating a firewall rule to give FileCoAuth.exe full internet access privelages and make sure a rule wasnt accidentally created to block it from the internet.