Musket of Thunder not working?

[Aurabora]

I recently picked up a bunch of cannoneers and the Musket of Thunder which has the text:

"+3 Attack of Archers, +20% damage of cannoneers"

I seem to be getting the +20% damage increase but their attack hasn't changed at all. They do have the 'Archers' tag under their info page so am I missing something or is this a known issue? Thanks in advance.