This is not a serious matter. Its more of an annoying type of thing. Shes not really being a brat, shes just being a rabbit 😄 Rabbits poop mark, some more than others. It’s an instinct designed to tell other rabbits that they’re there, this is their space. There’s probably a lot more additional info in the poop that we are not really capable of understanding, us being mere humans. Marking poop is often extra big and also shiny from scent gland oils.
The urge to poop mark often grows stronger during spring time. Spring (increase in amount of daylight) triggers the release of many hormones, not just sex hormones. Even de-sexed rabbits have some production of sex hormones though, the adrenal glands produces some testosterone in both females and males.
There’s not a huge lot to do about this, except pick up the poop and put it in the box and hope that she’ll catch on.
If she pees on her hay you could perhaps serve the hay not directly in the box, but within reach from it. They do like to munch while they do their business, and that’s a very good thing, a bun can never eat too much hay, as I’m sure you know.