After I broke Villager's workstation before trading anything with him, his profession remained. Even if I broke all workstations in that village, every single 'untraded' villager had its profession from workstation I built but after the spawned workstations were broken, their profession reset. I don't know how to describe it better, but I thing this is enough info about this bug (if it is a bug)
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The problem is that other villager has claimed that workstation, so the first villager can't use it or unclaim it (so he can lose his job) this can happend forever, meaning that you can have infinite villagers with job with just one workstation but only one of them can use it