Because when you opened the claim, you were getting Child Benefit so you met their standard to be "Primary carer". They based their advice on nothing more than your word and the child benefit.
You are no longer getting child benefit for one child so they have changed their view. Based on the change of circs.
Its fair because the advice you got aligned to your circs at that point in time.
Youre saying "its beyond amicable" but this whole time you initiated and pushed. And hes still said, he would sort it out privately. Despite the fact he is likely better off if this continues. For example if hes getting child benefit for your youngest, there will come a time where you might owe him CMS for DC2 but as DC1 is over 18 you get nothing back. He has said it doesnt have to go this way.
I dont understand why you keep having to put your hand in the fire. With every update you give more of us advise you how to limit the damage and you ignore it and say its unfair.
1 time, at 1 point in time CMS agreed you were primary carer. Based on your word and child benefit. And at that time that was accurate. But today he gets child benefit for 1 of the children, HMRC recognises you as 50:50 CMS now disagrees based on new information. Its not "unfair" its a different situation.
Honestly it will be so so so much better if you ring him and ask how he suggests you resolve it