Need to vent/advice

My SO and I have been together for almost 3 years. His daughter is 3 and has a developmental speech delay. I've been in her life since she was a year old. We moved in together when she was 2 years old. There is no custody order but they have a 50/50 split, where we get her from Thursday afternoon to Sunday evening. At her other home it is her and her BM - no siblings or stepparent.

Transitions have always been difficult for her. She used to cry every time she would get dropped off but now she's excited to see her dad and walks in happily. This is a big improvement!

However, now when she sees me she runs to hide behind her dad, asks for a hug, or starts crying. I'm not sure what her deal is. I don't want to take it personally because she's a child and isn't doing it to hurt my feelings. It does hurt to get rejected by her since I have done a lot to help her get acclimated in our home.

My SO is supportive and tells her that this behavior is not okay. She eventually warms up but it takes hours and now she doesn't ask to play with me. I am realizing that it is very hard for me to want to spend time with her when she acts like that.

Has anyone else experienced this? Any advice for dealing with these feelings?