Not sure how long this problem has been occurring, since I just discovered it today. Apparently After Effects has been rendering my 1080x1920 "vertical" comps at 1088x1920 instead. As you can see in the image below, it starts out correctly as 1080 in AE comp settings window, but once imported in Premiere, an extra 8 pixels has been added to the width for some reason. Luckily, If I change the Premiere sequence settings from 1088 to 1080, the extra pixels do disappear and the issue corrects itself. If I import the file back into AE, however, it still does show it as 1088x1920.
I've tested more 1080x1920 comps on our Macs and other PCs and this anomaly doesn't occur. This DOES occur on every single new After Effects project I make on my PC (Windows 10 Pro, 64GB RAM, 2 Intel Xeon CPU, 2x GTX 980 Ti), also occurring in CC 2015.3 as well. As I mentioned before, the fix is to simply revert the sequence to 1080 and everything seems fine. Not a life or death problem.
However, has anyone ever encountered something like this before? Possibly a video card error of some sort?
EDIT: Just found out we actually do have a separate identical PC rig that produces the exact same problem with 1088 on export.
This will be displayed on a 1920x1080 monitor, right? So the monitor has to be turned on its side, right? So why are you trying to make it VERTICAL? Make your comp at 1080x1920, then nest it in a 1920x 1080 comp.
Be sure you rotate the nested comp in the same direction as the monitor will be rotated -- clockwise or counterclockwise.
Did you solve your issue yet?