Someone else had the same issue last week and asked for some pix of my setup. I was away and didn't get any pix until today and now, cannot find that thread. Hopefully he is looking at this one too, as the issues are the same.
Here's a pic of a setup block (not necessary, but prevents test cuts), and the bit buried in it's cutout.
Horizontal piece, usually an end, just like a half-blind DT jig, lays flat.
This bit is a 45 degree miter bit with a locking feature added. Look closely here and you will see the top and bottom of the bit is lined up with the workpiece edges. Now note the top of the bit is in-line with the fence.
Cutting the side piece (vertical, like a half blind DT jig), supported against the tall fence and by a riding top clamp. I cut all but the very hardest wood in one pass. For 2 passes, there would be a second 1/4" hardboard fence face clamped to this one. Note the backing scrap on RH side that prevents chip out, and the whole thing is pushed pass the cutter using the rubber faced grout float. Once upon a time, I used an L shaped jig to push the piece and now I use a large carrier jig for very small stock. When the horizontal relationship is correct, the vertical relationship will be too.
The 2 pieces of scrap.
Fence pic. Also used for slot cutters and drawer lock bit (my personal favorite)