Dog doesn’t react well around other people?

I adopted my dog back in may of 2025. It’s just me and her most of the time in my apt. My partner comes over a couple of times a week and I just spent 3 days with him. Every time he comes over she still barks for a while and then goes to hide under the bed. When we’re at his place she is fine when he’s at work, for the most part will want to lay in the bed with me or walk around his room or lay in her own bed on the floor. When he comes home she goes straight to under the bed and hides. Why is she not getting used to familiar people? She acts like they are strangers. She doesn’t bite people or anything, she just barks a lot or hides. There are some times he pets her and talks to her and she’s chill about it but she is SHAKING the entire time when we’re not home or if other people are in the house. If it matters, she’s now 2 years old and I adopted her when she was 1.5 years. She’s completely different when it’s just me and her at home. She sleeps peacefully anywhere and loves to play and run around. Barely goes under the bed.