I don't have that style but...

My understanding is the RT is probably assigned in both the 'parent' (A/B) style as well as the multistyle (C/D). You can verify this using the style editor of both styles. If you assign a RT to a part and the multistyle defines a different RT, it will change it. You can get around this by using Bar Settings to change the RT or edit the styles in question. You can also use one of the blank styles if you don't want to edit anything.

R