I am so sorry for your loss, @Wensleydales . And for the callous and cruel way that your MIL has treated you and tried to (wrongly) make out that you were at fault in any way.
As someone said earlier, this isn't about generational differences. My grandmother was born in the 1930s - and lost a baby at birth. When I suffered a miscarriage at 10 weeks in my 20s, she went above and beyond to look after me, to make sure that I knew I hadn't done anything wrong, that sometimes nature is just cruel, that it wasn't a case of my body being faulty or me being a shitty mother (which my ex was telling me at the time), it was just something beyond anyone's control or comprehension. Compare that to my mother, who was born in the 1950s, when I had a miscarriage two years ago (14 weeks, needed medical intervention because of the blood loss and stage of pregnancy)... and this is a woman who has endured miscarriage several times herself, mind... and I was made to feel like I ought to crawl away into a hole and just wait it out alone and without any support whatsoever (she did likewise when I suffered a partial miscarriage at 9 weeks with my son's twin 19 years ago - so not really a shock this time around). I know, though, that she has unresolved trauma herself from her own losses, which she has never had therapy for. Actually, I don't think she's ever spoken about them to anyone - I only know because my Dad explained it to me when I had the partial miscarriage and couldn't understand her attitude towards (a) another woman and (b) her own daughter... But then, like you, OP, I have a great partner who stepped up and we went LC for a while until I felt strong enough to deal with her again. She has never mentioned our loss since, To her - it's like it never happened...
Being as kind as I possibly can right now, all I can surmise is that your loss has triggered something within MIL that you and your husband don't - and probably never will - know about. The fact that your husband immediately stood up against the way that she made you feel and told her how unacceptable it is? You've got a keeper, there.
I will never understand why it's always women making other women feel like shite when it comes to pregnancy - one way or another, especially with regards to miscarriage or stillbirths/loss shortly after birth/child loss at all... we ought to support one another. I know that my own experiences have rendered me more sympathetic to those of my friends who have gone through similar, to the point where I now consider my cousin's wife (who I barely knew at the time, but who I said "here to listen if you ever need a shoulder" to) more family than the son of my Aunt whom she eventually married!