Steps (hard to repro)

1- Open a large project
2- Make some searches
3- Close the project
4- While the project is closed, delete many files/assets
5- Re-open the project
6- Make some searches, the index will be incrementally updated
7- Try to import large packages from the asset store

Result: The search never ends or the merge background task and if you open Window > Search > Index Manager, the user asset index size is reporting 0 bytes
Expected: The index gets properly loaded even if some deleted documents were deleted.

Reported on the forum at https://forum.unity.com/threads/quick-search-preview.635863/page-16#post-8022893