Grandma triggered by parents calling the baby is theirs

I know this may sound super weird but my mom loves my little one so much that she got triggered whenever we mentioned that our baby is ours. Like when she gives us advice that we don't want to take, we will subconsciously say something along the line "let (me and my hubby) decide on our baby's care plan."

She would passive aggressively be like "sure, he's your baby".

I, on the contrary, have the belief that the more people love our baby, the better so I don't understand why this is. I guess I'm asking because I want to understand if this is a huge boundary issue that I should work on and arrange more care instead of letting her taking care of our LO at all so she doesn't get triggered by it.