Ominous banners obtained in 1.14.3 do not properly stack with ones obtained any earlier, in this case 1.14.2. I tried stacking one I had recently obtained by killing a captain outside a newly generated pillager outpost and found that it did not stack with my other twelve banners obtained a version earlier. When I placed down both the "old" and "new" ones, I found that there were no differences (same NBT tag identifying it as a white banner and same changes described in the item description). Breaking them, however, puts any collected banners into the "new" banner stack and any ominous banners collected will not stack with the pre-existing "old" banner stack.