Baby cries whenever with MIL?

My MIL helps out with the baby when I need to do my university lectures. She takes him for a walk or sits with him whilst I get on. However my baby (5 months old) gets distraught whenever he’s with her or she holds him. He literally bawls his eyes out and gets hysterical. He’s fine once given back to me. He doesn’t do this with my mum, he adores her. He gives her cuddles, laughs at her and loves being held by her. Why does he react this way with MIL? Is it just a phase? If anything I think he sees my mil more than my own mum but he just acts this way with her (and my partner’s dad). Any suggestions to help my bubba get over this? Can’t stand to hear him so distraught!