So, at the moment you have to unassign staff from a shop when you want to train them. However, after training, if you want to assign them to the shop, you only have two options. Unassigned or your first shop (For me the gift shop since I followed the tutorial). However, I cannot assign them to the second shop.
Hello! You’re right, good catch! Reloading the game should fix it.
Bug will be fixed on the next version, thank you for reporting it!
Reloading the game works.