The Vcore vrm is indeed not that great,
and it will definitelly not handle a 3900X / 3950X or a 2700X very well.
But a 3700X with just stock pbo should be okayish with this board.
100A ish of current draw should be doable with the board.
Of course i agree that a 3600X would have been a better match for it.
Because yeah the used mosfets are garbage. 
It´s indeed not a great board that is obvious.
And that also counts for the other ones i listed above.
The vrm of the board could play some role in the problem.
But i rather think that there is just something not right with the board in general.
I would recommend to take it back to the seller and ask for refund if possible.
Then i would recommend the Gigabyte X570 Aorus Elite,
if he could afford it.
Because that technically is a decent board.