With Kingery and offense I think the explanation is much simpler. There is a reason some players go through each level of the minors and often take a year or two at the higher levels. Kingery mastered AA but not AAA. He also learned the wrong lessons at AA with the easy home run park. When he got to AAA he never mastered the strike zone (look at his K/BB ratio) and also continued with his power heavy approach. Power is important to Kingery but more as a 15-20 HR guy with lots of doubles. Not a 30 HR guy that sells out too much and fails to use his speed adequately.
Kingery's approach was wrong and he never mastered the strike zone. Then he had a hot Spring (where stats are meaningless) and a big contract and got thrust into a major league role he was not ready for. While he improved his second year, there were still holes in his game that he never learned in AAA.
I don't really buy the hitting coaches messed him up theory. He was never developed properly in the first place. It is easy to blame everything on the Klentak/Kapler regime who made a bunch of mistakes. But they were not always wrong and the coaches were not always useless. Hey, we even promoted Sam Fuld after he produced the worst outfield defense in recent major league history, so it is sometimes not the coaches but the circumstances (i.e. what do you do with Rhys Hoskins who is not a LF).
I have no idea what to think of the effect COVID had on Kingery. Again it is an easy excuse. I just want the kid to relax, have a couple hundred low pressure at bats, and come up and help the ballclub as a good defensive player (whereever he is) that chips in with a 750 OPS and 20 SB. That player is probably worth $10 million a year and hopefully still is buried inside.