Example.
I have all profession tga files in a resource pack.
I only edited the texture cleric.tga, then I opened the game and applied the resource pack. When I spawn
villagers, I noticed that the Cleric Villager doesn't have the biome texture showing on it
The thing is, the .tga file actually stores some bits of code. If you have created a new .tga file, it won't have that bit of code. What you can do is overwrite the old one. Most of the .tga files in the game break when not edited properly.