I apologize if this has been brought up before, but I can't find any topics or posts on it. The topic title pretty much sums it up: even with the feat, the game doesn't register the shield as a second weapon on either the character or inventory screen, and a full attack action only uses the primary weapon. I found the shield bash modal button under abilities, but turning it on doesn't change anything. I'm utterly confused. Am I doing something wrong, or is this just a bug that Owlcat never got around to fixing?

I could take a look at your Character sheet if you wish, but if you meet the requirements of the feat and your Character is not impaired you should be able to use it.

it is best to report it as a bug.

I may have found the reason. I bought the magic buckler from Joran Vayne in the Defender's Heart yard. It turns out that bucklers are a separate class of shields from light and heavy shields, and Shield Bash specifically mentions 'light shields and heavy shields'. Switching back to a generic light shield lets me bash again. Seems like an odd distinction to make, but there you go.

Yes, there are different types of shields; bucklers, light shields, heavy shields and tower shields.

A 'bash' attack is only for light and heavy shields.

I believe it also requires toggling an ability granted by the feat. Just picking it at level up is not enough.