Many of the lemons when cut have a brownish pithy pulp and sometime green seeds. I find this in fruit that appears ripe and some that still have quite a lot of green. What causes this?
Be sure to fertilize your trees often with an all-purpose fertilizer. They may either have too much nitrogen, or not enough phosphorus. Here is an article that will help: https://www.gardeningknowhow.com/edible/fruits/lemons/lemons-stay-green.htm