Set Initalization Data for one equipment item (like a bomb for example) and activate quest rule: Can select A button item in subscreen.
Now save the quest and test it. You will see the equipment item in both buttons.
It should be only one item in A button at start and can be swapped to B leaving A empty and back.
It must be something to do with the first postion in equipment (Position 0).
EDIT: I tested and replicated it. It is something to do with Position 0 and that is what causing. Here is the test file to load in ZC, note that bombs is first postion but can be all item classes. Look in the active subscreen in ZC.