Windows search is very much broken.
I have a few instances of file explorer open. In a few of them I can search any file and text string in any folder, local c drive, my local data drive as well as mapped drives on other systems on the lan.
This works with OR without indexing enabled or windows search service running.
If I open a new instance of file explorer, that instance will NOT search any thing any where.
I have tried all the "normal" listed fixes that are nothing but a waste of time.
sfc
troubleshooter
ending cortana
stopping and starting search service
enabling and or disabling indexing
verifying none of the drives or folders in question are excluded from indexing for times it is enabled
enabling or disabling of the open files in new window folder property
enabling or disabling of including filenames or content of non indexed folders
Notice I stated clearly, in some file explorer instances I CAN search and some I can not, same folders etc. In the instances that do not work, the search box shows, just does not allow imput
Now that is a true "head scratcher" given all the things you've tried. I think it's pretty well established that this is a genuine problem, whether Microsoft chooses to acknowledge it or not...