Firstly, my deepest condolences for the loss of the baby. It has never been easy dealing with the demise of your loved ones..at least for me.
Newae, back to e situation. I feel that there might be a lack of communication between daughter and her mum, be it generally or about baby's passing.
From my counselling background, I understand that different people face/handle grief differently and in this instance, mum may have chosen (perhaps subconsciously) to put up such a front in order to be strong for her daughter.
It's difficult to share coz I dont really know e actual situation but maybe mum has her own emotional baggages from the death of baby that she never did deal with completely and maybe also that's the reason why she felt that daughter should have not posted the update as she sees it as a family matter and should be kept private.
Mum and daughter should talk it out amicably and maybe it will help to go for counselling together as the counsellor will be able to act as the facilitator and middleman as both mum and daughter are guided to walk through their emotions and overcome their grief gradually. Just my humble opinion..