Cured Zombie Villager Switched Professions Well After Trading with him

A few of my friends and I had captured a naturally spawned zombie villager and had cured it. According to the internet, they are supposed to keep their profession, but after a while, they changed it (before trading). After I leveled up the one we made into an armorer into an expert and traded for a couple of diamond boots, out of nowhere, he changed it to a weapon smith, and he hasn't changed back. I've broken all workstations, and he just randomly changed professions, and I'm not sure why. Our other naturally spawned zombie villager that we caught and traded with did nothing like this; however, he also changed his profession after a while before trading with him.