No, the way it used to work is it would try to cast something, if it was on cooldown it would say "oh well" and "roll" another skill with the next push. Sometimes it would pick the same skill, other times it would pick something else, as it was an entirely new randomized attempt.

From my understanding {I don't have any castrandoms to try atm} it now refuses to move on or "roll" a new random attempt until it casts the skill it tried to cast...exactly like a castsequence that gets caught on a cooldown.

For example, my hunters used to have :

/castrandom Kill Shot, Kill Command, Rapid Fire, Steady Shot, Tranquilizing Shot, Viper Sting

as the tail end of a /click macro. At random points, it had the ability to throw those abilities in as filler. If, say, a mob didn't have mana and it tried to cast Viper Sting, it would throw an error message and then ignore the cast attempt and next time around it could try casting something else {or Viper again, depending on luck of the draw}.

Now, it would try to cast Viper, say "I can't cast this" and keep trying to cast it every successive push saying "I still can't cast it", like the guy that walks into a window and says "Hmm, I can't get outside" and proceeds to keep walking into the window.