All screengrabs are from
1998x1080 "DCI Flat 1.85:1" DCPs
created with the specified size footage in the specified size Premiere sequence
exported directly from the sequence using the stock Wraptor DCP setting in Adobe Media Encoder CC2014

  1920x1080
1.777:1
source footage		 2048x1080
1.896:1
source footage		 2048x854
2.398:1
source footage
1920x1080
1.777:1
Premiere sequence
Cropped top & bottom!?!
1998x1080
1.850:1
Premiere sequence
2048x1080
1.896:1
Premiere sequence
Cropped left & right!?!
Cropped left & right!?!


FWIW, all source footage that didn't match the sequence frame size was adjusted using "Set to Frame Size" in Premiere. (As opposed to "Scale to Frame Size" or manual adjustment in the Motion Effects Controls.)