I had several kernels I was selling to buy orders. For this specific kernel, there were 2 player buy orders for the same price. 1 of them was buying 49 and the other was buying 1. The order of 49 kernels was a the top of the list of buy orders in my market window.

When I tried to sell the whole stack of kernels, I could sell a maximum of 1 kernels. I did that, and then tried selling the stack again. Yet again I could only sell a maximum of 1. It turns out that I was actually selling to the buy order of 49 kernels, but there seems to be a different sorting when it looks at available buy orders when trying to sell.

Luckily I only had 10 kernels, but it's annoying when you'd have 100+. A better option might be that if you choose to sell a stack of items, it sells them to the best available buy orders and tries to sell as much of the item as possible, i.e. until the buy orders for that item have run out.