That can be a problem. If you're using a flanged insert, can countersink it ever so slightly, just enough to make the underside of the high side sit flush with the riser. Of course, the top of the low side should still sit proud of the riser wood. Talking 1/32 or less here. If more than that, maybe the riser isn't a good candidate for inserts.
I have a 2017 BW catalog and notice that the inserts are no longer listed there either. May need to contact BW directly.
One other thing to keep in mind. The inside diameter of BW inserts, about 5/32, I think, is smaller than most other inserts I've seen and used. The latter are usually 1/4-inch inside diameter. So if you want compatibility across different risers, be sure to use the same inserts on all of them.