You are mistaking suspicion with proof. Even if the voter registration database was deleted, that is circumstantial. It doesn't actually prove that voter fraud occurred. I would agree that it is very suspicious if there is no reasonable explanation, but of itself it is like saying a missing receipt is proof that something was stolen.
Missing voter registration databases would actually make confirmation in either direction difficult, so it could have just as easily been deleted by someone trying to make it look like there was fraud as someone who wanted to cover fraud up.
That also assumes that there are no other records for voter registration that could be used to rebuild the database.
If by research you mean a tweet by the twitter audit account, then sure. https://twitter.com/arizonaaudit?lang=en
They said the database was deleted based on the file being deleted in the MFT. That's pretty sound, however, from personal experience, applications can be odd and complex. There are often temporary databases that are created and deleted all the time. People do strange things like move databases around, increment them, etc. If you move a file from one drive to another the MFT on the origin drive would record a delete and the drive moved to would reflect a creating it.
Some databases are also created from other databases, so it is possible that as part of the power down process that database is deleted or moved to the system drive and then it is recreated/moved from somewhere else at power on. Since the forensic team identified that they recovered the data from a powered down state, the file could be missing but would have been present had they looked at it in the running state.
I'm not saying that is specifically what happened, but there are reasonable explanations.
Regardless, both the twitter account and the video above state that the databases were recovered.
https://www.azleg.gov/videoplayer/?eventID=2021051030
You can go here and go to 00:48:40
or use this direct link:
https://player.invintus.com?clientID=6361162879&eventID=2021051030&startStreamAt=2920