I once tried out a villager spawner mod, and discovered there is a traveling merchant profession vanilla villager, but when I reloaded the game, the villager turned into a farmer villager.

Hmm all vanilla villagers that get the traveling merchant profession should become farmers yes, however this should happen immediately and not only on reload. Will check, thanks!

This cause another problem, sometimes there will be a farmer that have the first tier trades of a merchant, can you make the merchant profession not appear in villages, or in breeding, spawning villagers?

That last comment kinda makes sense. Will check

Will be fixed in the next build :)