Here you can see my hearthling was level 6 footman and changed to herbalist.
Now he needs a looooooooot more Xp for the first level up.
It is like leveling up a level 6 footman to a level 7 footman.
Is it a bug or not that someone needs more XP for a job level after leveluing up a different job?
This is intentional. Each time a hearlthing gains a job level their “total level” increases, experience to the next level is based on your hearlthings total level. It is designed this way to stop players from just changing jobs for hearthlings all willy nilly.
The hearthling you have there is a level 0 herbalist but is actually level 6 due to the levels gained as a footman. So the experience needed for lvl 0 -> lvl 1 herbalist is actually the experince needed for your hearthling to go from lvl 6 to lvl 7. I hope that makes sense.
This is mainly there to stop you getting a level 90 knight with ~1300 hp