I had several armorers and smiths with really good armor/weapons switch to farmers when I put down composters. Meanwhile jobless villagers never choose a profession, and just wander around aimless, even if I put a new job block in front of them. If I break an existing job block, the villager immediately switches profession while the jobless remains jobless.

All of my existing villagers with established jobs of which I regularly trade with just randomly switch if I put down new blocks instead of jobless villagers claiming them. What's going on here?

I've spent countless hours grinding for trade resources to upgrade their levels and get decent trades, and then they just switch jobs spontaneously. How can I stop them from switching jobs?