I'm not printing borderless and have Sheet selected. I'm using a custom size, 8.5x11 with no margins
Well... that is borderless as far as the printer is concerned... even if your actual image area doesn't cover the full page. Your page setup tells the printer make a page where I can print edge to edge on all four sides (no margins) to a letter size sheet of paper. A 9900 can't do that (I just looked at one) and doesn't understand that page definition so it goes back to its defaults which is what I described earlier.
There has to be margins on a letter size sheet of paper. The largest minimum of the default margins is .56" on the trailing edge. You can't make that any smaller. Therefore, keep that one at that setting and increase the leading edge margin to the same .56"... and your work will be centered. However those dimensions are going to result in maximum long dimension of 9.88" of image area. 10" won't fit.