is auncle really that controversial?

so my sibling is having a child soon, and I've been out as trans/enby for years and have called myself auncle to all their pets up until now. but recently they told me they're not comfortable having the child refer to me as auncle, and hope I respect their decision (fair) as much as they respect mine (i.e. regarding my gender). what I'm wondering is if any of y'all have any experience with the term, and if it in your experience has been hard for children to use or comprehend? cause I cannot for the life of me understand how they're "ok with my identity" while saying it will under no circumstances be ok to refer to myself as auncle and I have to choose between uncle or aunt?

edit: my native language isn't English and the translated version of auncle is pretty easy to distinguish from uncle/aunt