Hmm, yes, I noticed this too... it seems strange that the card picked for forging is always locked; but the stats gained when you use bexp are not (however, it is considerably more likely that you'll get a high growth than a low one as one of the 3 picked, for obvious reasons, which can from time to time make it seem like its locked; but my hour spent ensuring Micaiah got Speed in three level ups wasn't a total waste

)
Of course, if you do have coins to spare, and want to move onto the next card, you can simply forge a trashy no bonus weapon and use it as a normal one (like a Steel Lance or whatever).