Cannot moon grind with claptastic legendaries

I have found a problem though with the claptastic legendaries and not being able to do moongrinds.

I tried grinding a luck cannon (claptastic legendary). a skullmasher and a purple shotty. It wouldn’t give me the option to moongrind, just doing a normal grind.

Also saw another wierd thing which probably contributes to this, the luck cannon has a much lower price than a vanilla legendary of the same weapon type and level.

It seems as though the game is not reading the claptastic legendaries in the same way as the vanilla legendaries