Does anyone find that spotlight forgets docx files every day?
I was having a similar issue with Numbers files, with them being remembered by Spotlight for varying lengths of time and then being forgotten. Often I had to open files from the 'Open Recent' menu because Spotlight didn't return them. Usually that got them back into the spotlight index, but only temporarily. Perhaps it wasn't only Numbers files, just that those were the ones I'd been searching for.

In my case, it turns out that I had very little free disk space. Enough for what I thought of as my storage needs, but apparently not enough for the system to operate correctly.

I moved 30GB of files to an external drive, emptied the trash, rebooted, ran sudo mdutil -E /, waited for indexing to complete, rebooted again.

That was yesterday, and so far so good. Spotlight has returned everything that I've thought to search for. Also, my machine had been getting quite sluggish but now feels quite responsive. I hope this helps you, or others who find this thread.

[Update: things are better but not solved. There's still some mysterious gaps in my spotlight search results. Doesn't seem to matter whether it's files on the local disk, on iCloud drive, opened recently, etc.]