I don't think it's the stem, but the length. I run a 90 as well and the mount is just too long to tuck into the space available. I'll likely be going accessory bar mount or adding some sliced hose to lift the mount off the stem. You can see the type of hose I use in this photo, where I used some under my harness straps.
IMG_2245 by
mbeganyi, on Flickr