Possible to add smart features to a flexible assembly?

I have a sub assembly for a pair of drawer slides (left and right since they are sold in pairs). The slides require holes to be put in both the drawer front and back of the box for fastening. I made the sub assembly so it could be flexible to accommodate any drawer width that we might need. I also made a dummy assembly and added the smart features to drill the holes required.

Every thing works fine until I make the sub assembly flexible. As soon as that happens the smart features no longer follow the part that can move. Is this correct or am I doing something wrong? I don't see any options that I'm missing.

Is there a reason you have to make the sub-assem flexible? That will just allow the sub assembly components to move in relation to each other/ follow their internal mates. If you have two drawer slides they will move as one when bolted to a drawer right?

Yes, but the slides are sold in pairs (left and right sides). I have limit mates in the sub-assembly to allow for the proper pull out lengths, but I have one side fully defined to use as an anchor and the other side is only constrained in two dimensions. When I make it flexible I can mate the loose side to the cabinet frame to work with any drawer width I needed. The drawers are linked to the cabinet so they can adjust as needed according to the frame size.

In this image the slides are fixed and this is where the holes would go if I added the smart feature.

In this image, the slides are flexible, but when I add the smart feature the holes do not follow the slide that has moved. They are in the original position.