Sometimes girls just can't poop right and they just have dirty butts.
If more than one or two have dirty butts you may be dealing with worms or vent gleet.
A poop test should confirm or deny.
I read the info you attached. Sounds like I'm in the right track for vent gleet, just need to soak and trim. But need to rule out parasites. Their poos are super healthy looking but I'll have them checked to be sure.