I think why he got angry with the ease with which she was around ACP Was probably he knows that she can fight back when she doesn't approve of someone touching her inappropriately- AA force kissing track. The look he gives her of admiration and approval when AA admits she slapped him.
Yeah intially till the MU happened, it was like he was not able to understand how she is so comfy with him on Day 1..😆
But then MU happened , i guess he even started feeling something for her by then, and it increased 10 folds.