Our large BJG had something similar. We have laddder style roosts, and had the 2x4 roosts turned wide side up. This was a mistake. The keel bone of the BJG rubbed against the wide roost (and in winter they are on the roosts more hours of the day) and the feather/skin/keel bone/roost bar interaction caused a breast blister. Then puss entered the scene. Chicken puss is like yellow firm cottage cheese. It is not thick and runny like you might think.
So we discovered the breast blister. And posted on here to get some advice. You can search my posts from Jan 2019 to find it in the emergency forum.
we soaked it daily in a warm epsom salt bath in the utility sink. Big rooster needed a deep big sink. After we soaked him daily for 15-20 min, we dried him applied Neosporin ointment, applied a gauze pad, and black vet wrap (he was a black rooster). After a few days it softened we could remove the scabs. We did have to use a scalpel to make a small opening and squeeze out some of the puss. At this point we also used iodine snd sterile contact lens saline to rinse and treat the skin. More ointment, gauze, vet wrap.
we treated him for several weeks. It got all better!
We moved all roosts to narrow side up (except the bottom one bc no one sleeps there). Never a problem since.
my guess is that even though your chicken isn’t as large as our BJG, the frizzle feathers may have added to this occurrence bc of their shape/type.
good luck!