I have been using the built-in Google geocoder from CiviCRM for ages (since 4.x). After upgrading to 5.5.x, I noticed it no longer seemed to work, even after setting up a Google Maps API and getting the correct API key. So, I then installed https://github.com/eileenmcnaughton/org.wikimedia.geocoder and attempted to use it. At some point during using this extension, many of my addresses had their STATE field completely wiped out - with the zip, street address, city, country - still intact.
The extension says it is able to fill in missing details on the address from the geocoder, so I'm assuming something like this was attempted but failed, thus resulting in some state's lost. Does anyone have any other suggestions for what might have happend and how to fix it? Yes, backups are available.