Obscure, but quite easy fix:
https://developercommunity2.visualstudio.com/t/find-does-not-show-results/954875
HKEY_CLASSES_ROOTWow6432NodeCLSID{73B7DC00-F498-4ABD-AB79-D07AFD52F395}InProcServer32
After removing the extra backslash in the path, Visual Studio Enterprise 2019 Version 16.8.3 suddenly started showing search results in the Find window.