We really need to look at LARGE sample sizes to determine stability. So much variation can be environmental rather than genetic, even when the plants are siblings in the same tent.
But let’s put it another way: if we had a huge farm filled with black Labrador retrievers, we would see yellow and brown colors emerge after a litter or two. Is that breed not stable? Of course it is. It’s just stable for those 3 phenotypical expressions.