The recommended procedure is to install a vapor barrier behind the cbu and lap it over the tile flange. You can adhere the tile with thinset IF at least half of the tile is on the cbu (i.e., if the tile is over 2x1-3/4" in your case). If the tile is too small, then you'll have problems. While a little messy, you should be able to grout that area.
Another possibility (too late without a lot of work) is to notch the studs so the flange is flush with the stud face, then run the cbu down to about 1/4" from the top of the tub (over the flange). Or, you can shim the studs then install the cbu over the flange.