So I was wondering some more and the only thing I can think of on why her yolk may be THICC is due to something she is eating. Does she get into cattle feed? That is high in cottonseed oil which can make a thick yolk.
It could also be a hydration issue, is she getting run off from the water? You said she laid a few thick yolks in small eggs over winter, which isn't unusual, the thick yolk part anyway. The strangeness is the fact they are such little eggs.
Otherwise... If she gets plenty of water, isnt picked on, or doesn't get into the cattlefeed, that is just how she is. It may cause issues later but not sure how much you can do.