It might help if you think of the holy orders and sacraments through analogy, although in truth I don’t like them.
Let us imagine that you have a license to practice medicine in Timbuktu, or maybe China
You come to California or Illinois and you are not allowed to practice. You need to be licensed, and that is predicated on a lot of things. The state does not have an opinion on the ability of Timbuktu to train and license an MD (individuals might have suspicions), it doesn’t have an opinion on your ability as a healer either. It must behave as if you have no license at all and you may have to prove yourself through examinations, and possibly retaking some courses.
Then you will have a license from Illinois, and what other licenses you may have had before would be completely irrelevant to Illinois.
It’s the same with a drivers license. You might have a license from Georgia, and that would get you permission to drive in Illinois passing through, because the states have a mutual understanding. That does not necessarily mean you would be allowed to drive in another country. If the country says no, they will not be passing judgment on your ability, or the ability of Georgia to test and certify you. They will simply be saying “we don’t know” and require you to prove yourself through a test.
Orthodox only certify Orthodox men to be priests, what other churches do is of no concern and irrelevant.
This holds true for bishops as well.
For Orthodox, a man has to believe in Holy Orthodoxy entirely, no more and no less. A man claiming to be a bishop while outside the church is completely irrelevant. He needs to believe what Orthodox believe, have an Orthodox following and be in communion with other Orthodox bishops. Lacking this, he is nothing to Orthodox. The Orthodox churches do not have a theory that someone could be a valid bishop outside the church, which is why it is not possible for Orthodox to recognize Apostolic succession in the Roman Catholic nor the Anglican churches, and vagante movements are not recognized (Thuc, Vilatte, Costa etc.).
Likewise, if an Orthodox bishop should leave the church, he is no longer a bishop.
He cannot take it with him nor pass on the role to anyone else.