Not sure if it IS a bug, but the 'non-combat' wildshape forms (Deer/Cat/Mink/Rat/Dog/Ect) can still be used, even if the regular wildshape charges have ran out.
Alternatively, if that's how it's supposed to work : the message 'this ability is tied to your wildshape, and it has run out' plays whenever you use it when the wildshape has ran out.