Try reordering your masters so a different one comes first. It is a bug with
the master slides -- I *think* it's that the formatting of the first master
in the list gets applied. Or something like that.

That might mess up some of the other slides, though. If so, simply copy the
text placeholder and paste it on the slide. Delete the original. Now the
placeholder isn't tied to the slide master, so the formatting should stay
put.