Sweet potato allergies are real and they happened to our baby too!
I've started introducing some solids and peas and sweet potatoes were among the first veggies I fed our baby. A couple days after trying peas she was fine so we tried the sweet potatoes and she started spitting up. And spitting up.... and spitting up. Very unusual for her, especially since I made sure to burp her afterward. I thought it might be something in my milk since she's still nursing as well and has various allergies when I eat dairy, fish, etc. The next day I made sure to avoid anything that might aggravate her allergies and when I gave her sweet potatoes she started spitting up again (and again and again) and her eczema, which gets bad when I eat something she's allergic too, got worse.
So I decided not to give her sweet potatoes again. But just in case we were wondering whether she was REALLY allergic, I went to work one day and my sweet husband accidentally fed her some of those potatoes instead of the squash ("They're both orange!" and the Gerber plastic tubs of baby food aren't individually labeled unless you look very closely) and we spent the next few days dealing with her worsened eczema and spitting up.
I definitely think sweet potato allergies exist in babies. I'm not sure what it is about them, but I hope they grow out of it.