Sometimes in my village, it seems an unemployed Villager claims a job site block that had already been claimed by another Villager, creating more jobs than there are job site blocks. (In the case of the screenshot, more Farmers than there are Composters.)

It at least has a relatively simple workaround. Just trap one of the two Villagers that claimed the same job site block so they can't reach it anymore and become unemployed. Then you can release them again and they'll realize they can't reclaim that same job site block and stay unemployed.

I am not sure what's causing this, but the village is typically unloaded (due to my base being too far away) and only gets loaded when I consciously visit it. Maybe it has something to do with unloading and reloading the chunks the village is in? Pressing F3+G shows me that these Composters are both in the same chunk, but the village itself is spread across several chunks. In particular, the beds of these Villagers are in a different chunk than the Composters.

As you can see in the screenshot, it shows three Farmers around two Composters. There are definitely no other Composters in the vicinity. Using the workaround described above, the third Farmer successfully became unemployed and stayed that way.