That said, was this a clean install of each insider build or upgrades?
The update that seemed to coincide with Start10 not being able to access search was actually neither, technically. I did an install in place, so Windows retains my application and files but is much more thorough of an install than an update, but resets less things than a clean install.
I understand it's difficult to diagnose something that seems connected to Insider builds, however I'm not sure MS is testing for Start10 compatibility so not sure this will ever be addressed on their end. Since native Windows search works entirely as expected outside the context of Start10 but no one else is reporting/replicating this behavior, I suspect something has broken in my Start10 installation that is not being corrected by an uninstall and purge. I did notice that in the 2 times I used the Purge script, there were still remnants of Start10 left behind in directories, the registry, and the indexer settings. That makes me wonder if enough things have been reset back to a pre-install state before re-installing Start10.