My 6 month old exclusively breastfed baby is rejecting formula milk and is forming rash around mouth

Reason for introducing FM is bc I was down with stomach flu recently and my milk supply dropped ALOT bc of dehydration. Struggled to feed her even by direct latching. Hence, husband and I had to supplement her with FM (adviced by doctor too as i was on medication). However, baby refuses to drink the FM. We tried Lactogen (she drank this when she was a newborn at KKH, then all the way BM only). Rejected. Had abit of a rash around her mouth when she barely drank it. Tried Kendamil, also refuses to drink. Abit of the milk got onto her mouth, and again. Rash. We’ve tried Karihome, she has drank this abit when she was around 1 week old at home when i was having my massage and didnt pump extra. AGAIN. The rash around her mouth. No rashes on body, no other symptoms. No diarrhea, no vomiting. Rash goes away after 15mins. Should i discontinue the FM bc the rash forms around her mouth? Or am i still able to continue? We’ve had abit more luck with Karihome, but then again the rash appears. Any advice from mummies? Any of your LO went through this?