Powder vs. RTF sensitivity/allergy

My baby is 4 months and has only had breast milk except for the day after birth when he was given the ready to feed bottle of similac 360 total care at the hospital (which he had no problems with). We just gave him an ounce of the same formula but the powder version to try for the first time today and his entire body broke out in to a rash. Has anyone experienced anything similar or does anyone know what might cause this problem with the powder but not the RTF?

He was seen at urgent care and we will be following up with his pediatrician of course so I am not looking for medical advice or “see a doctor” comments. Just curious if anyone else has gone through the same thing and what adjustments had to be made. TIA!