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600 was sadly the border between garbage and low. By going up 30 points to 630 you passed the stable low number of 620. Basically while we all understand that you had problems in the past your score became solid enough to not be garbage. In short you definitely have moved out of the subprime market to a more preprime area which while not great will move your percent by a reasonable amount that results in thousands difference. I would encourage you to go shopping but if all else fails you know you have one company willing to loan to you on less than ideal terms.
Lenders can, if they so choose, close at near light speed if everything is in order.
People can and do switch lenders for a whole bunch of reasons, and in this case a takeaway often is one of those reasons that a bank will get up off it's butt and UW quickly... like seriously it can be closed under a week certainly by some lenders including the ones I've worked at in the past.
If it were me and I had a non-trivial credit score change that made a difference on my mortgage tier, I'd absolutely be all over socializing it with the current and new lenders if my current wouldn't play ball.