FWIW, I find if I’m pushing the disk limit, Spotlight search becomes unusable. So I free up ~100GB (easy to do with iOS/Android dev artifacts, emulators, etc.) and then force a Spotlight re-index from the command line:

sudo mdutil -E /

You can check indexing status with:

mdutil -s /

After that it’s usually fine again until I start pushing the disk limit.

Every sibling comment's issues I've experienced. But a reindex after freeing enough disk space seems to be a semi-stable solution for me. If only I could write a tool to remind me to free disk space before it screws up spotlight. It would be nice if MacOS put the index on its own partition.