This bolded is an odd statement. As someone else mentioned babies do not have concept of time or even of "object permanence" The idea that mom is out of sight but coming back. (That's why we play peek a boo with babies, it's a fun silly game, but they learn object permanence from it)
Has this pattern developed over time? Or from birth?
I worked in infant child care. I was shocked to learn that there are some babies who could literally cry & scream all day. BUT many of those babies started crying at drop off & never stopped. Rarely did i have a baby get dropped off, fine for an hour & then be sad for the rest of day (consistently)
This tells me something- the time that she is calm, you are calm, (maybe relaxed) playing with her. Then something must happen to break the engagement. Do you follow the mother's routine strictly? What is triggering the crying? Do she have a favorite activity, that you could do? like bath time or feeding, or snuggling? something just isn't making adding up for me. if she is familiar with you & bonded to you, this shouldn't be occurring every time you watch her... if you wrote & said the first 30 mins are filled with crying and then...That scenario makes sense. but flipped, it doesn't add up.
Why don't you wear her? Have mom show you how to wear her.