we weren't speaking for a while. And that's the way I liked it. But DP didn't like that and it caused so many arguments. Me and the MIL ended up speaking again
That's actually a potentially useful information nugget for you.
Set aside for a second the rights and wrongs of how the 2 of you ended up not speaking. Focus on the cutting off of contact bit because it might be relevant.
She bonded with her son's children. His relationship broke down. Like a lot of grandparents she might have had concerns that her relationship with her grandchildren could end up as collateral damage. And she could have had concerns she might be left grieving for children she has a strong bond with, that she is allowed little to no contact with. It's not a baseless fear when your adult child's relationship falls apart.
So far she has had a reprieve with the mother of his older children. So far. She might not feel she has any guarantees. She might worry about the future and their mother moving away, moving on, taking a different view of such a close relationship with grandma being something she will allow.
Once bitten a million times sub consciously shy. She might, without any malice or even conscious awareness, be avoiding bonding to the same extent with new grandchildren. Because she looked into the chasm of of potential loss, and can't face any risk of feeling like that again. Not creating the same sort of bond with her second wave of grandchildren might have been about building a protective layer around her heart, not something personal to you or your children.
But with one bout of not speaking and a forced reconcilliation from your perspective already cropping up, any "self protection from loss" strategies could kick up a notch when it comes to your children specifically.
I'd seriously suggest looking at the whole picture, including the ex and her other grandkids, with her lens. You won't find the key to resolving an issue, that is obviously and understandably upsetting for you, until you can take a 360°, unbiased view of her potential motivations. Her reasons could very easily not be bad, they may be sad, and scared. If that's the case, friction and antipathy between her and you will not make that go away, it will only create so very much more of the same with knobs on.