Absolutely it doesn't but they were teaching her but not what you wanted them to teach. You wanted differentiation which is not what happens in teaching any longer as it limits children. Maths is taught to get progressively harder so all children can achieve. You think that not having differentiation means they are all taught easy stuff but that is not how it works. Questions start off easy and get harder so it incorporates ALL the questions from the previously differentiated work. So your child is still getting the harder stuff but it comes later on as she has to show she can achieve the basic levels before moving onto the harder questions which would've been the higher ability of old differentiation. So your child is still being exposed to the same thing but you are not understanding how it works.
It is not taught how we used to have it with questions that are all on the same level. You are assuming they are. They are most certainly not. 4 + 6 = __ is easier than 🔶+ 💛 = 10. The 4 + 6 question would be early on and the square + heart question later on.
Your child being able to do sums in her head doesn't show she is secure in something. It shows she can follow a process and get an answer. Maths is not about getting an answer. I can rattle off my times tables but that is not doing maths. There is no maths involved in saying numbers.
I am sorry to hear you had a bad time and I can imagine it was a nightmare but blaming others for you wanting differentiation which is not done any longer so assuming that she is not getting the higher level work is wrong. All get the higher level work now they just have to complete the easier work first. If your child was struggling with the easier work then there might've been other reasons for that.
Maybe your child was struggling with what was going on with her sibling and that was the main issue causing her to struggle at school as a worried child cannot concentrate properly. It would make sense that a child would not be able to concentrate at work if they are worried their sibling would die while they were there.
*Just saw your edit where you falsely accused me.
Absolutely teachers can do wrong. I have done wrong as a teacher. All do things wrong. What you cannot grasp is that you want differentiation but do not understand that removing that doesn't remove the challenge - that is still there for those who grasp concepts rapidly. You have assumed they set no challenge but that is what maths is all about and the curriculum elements are about challenge and challenge is the whole reason why differentiation was removed.
So yes teachers can do wrong and do do wrong but just because you do not understand the curriculum doesn't mean they did.
What has happened though is your child struggled when your other child was ill and you say the school is the issue. Look at what links here.