When the new Class Update version 4.2.5 released on the 15th of Jan, all of your Teams were automatically assigned a class. The team you are using the Weapon in may be a different class from previously. You can learn how to change the Class to the correct one here.
Do you now have the Shaman Class equipped to the team you are using with the Weapon, and if so, is the issue still occurring?
@Lagertha, when were these screenshots taken, i.e. after playing what kind of battle (Pet Rescue, PvP, etc.)? Were you accessing the Troops menu from the World Map or did you go into Edit Teams before an Explore battle (e.g.)?
I have a feeling this may be related to my post in this Bug Report thread:
Edit: I think it’s particularly note-worthy that @Lagertha’s Hero has no class attached to it in the 2nd image, which was apparently taken after the 1st, and despite the Shaman class being visibly assigned to all of their teams in the background.
I’m thinking perhaps you played a Pet Rescue (or Delve or similar), without having a class assigned to your team (as they didn’t automatically apply), which could have caused your World Map avatar to have no class attached to it(?), which meant that clicking on a Weapon in the Troops menu caused your Hero to show up without a class, as per my post in the other thread, and thus without the bonus damage.