I have a friend who was like this, always slightly detached from her children, when she and her husband eventually split, there was no custody battle she didn't want her DC full time,
she only has them a night a week and often forgoes that, her ex, nice guy, is very none judgemental about it, he said he realised as soon as they had them she wasn't maternal,
He begs her to spend more time with them,for theirs,his and hers sake, she just never manages it, the oldest ,12 year old is starting to recent her,
she lovely woman, has a skilled medical job, she just not into being a mum, dads opt out all the time, sad for the children, never pays maintenance,
I'm sure if we all were harsh towards her she would of done a full bunk, years ago, her DC are far better seeing some of her , than nothing, so we try to be encouraging, invite her to the fun stuff,
I wouldn't of left any of mine, but that's me, I don't expect anyone to live my life.