We all told you over and over that CMS will award on the basis of child benefit and on what you tell them. They don’t check. They rely on honesty.
You only got the child benefit because your ex allowed it as a goodwill gesture. It’s 50/50 care so should have been one each since 50/59 started. Your ex allowed you to keep both out of his goodwill. You weren’t actually entitled to both.
You then used the fact that you got the child benefit to claim maintenance. We all told you that CMS would award it because you got the child benefit, that’s how it works. Your ex put in his rival claim for CB because it was the only way to stop you. We told you that your ex would win his rival claim, and you didn’t believe us. Look what happened. He had to do that because CB is the proof that CMS use. He got that proof for himself so that’s how he got one child removed. It is 100% correct that they have backdated that removal to the day he was awarded the CB so that’s why it has been deducted from the payments for the remaining child on your claim.
He will now win the claim for the other child. You will have to pay him. If he chooses not to accept it then you are very very lucky.
But you need to stop saying CMS are at fault. They go solely on what you tell them and on the CB.
You were not entitled to both CB claims. You only had both because your ex allowed it out of goodwill. You abused that goodwill when you used the CB to claim child maintenance. It is not the CMS fault. You knew you only got CB because he let you, when you abused that, he took it from you. CMS only make a decision on CB so they said you were entitled to money because of that… but you knew you only had CB because your ex let you. So you knew you weren’t actually entitled to it, which leads onto you not being entitled to CM for at least one child. CMS didn’t know that.
This is all your own fault. You deserve everything that has happened.