Help! Underside balding

Evening everyone,

One of my hens has recently started balding on her underside and I'm not entirely sure why, she is the only one of three that has anything like this, she is approximately 18 months old and she is the largest of them (if that means anything) I think she's just a hybrid breed.

I can't see any mites or lice (although I realise not all are visible to the naked eye),

there's no unusual smell,

She is laying as usual, although recently layed more soft shelled eggs than usual prior to this happening,

She's not being bullied or excluded by the other two,

She's eating and drinking as usual,

Dirty feathers near-ish her vent (as pictured)

Poos have been a little runnier than usual but I suspect that's because we are in a heatwave here (UK)

Hope I've covered everything and would appreciate any thoughts and knowledge from you all.

Thanks in advance