Adobe Premiere Pro Export not full screen

Hi, appologies if this is a dumb question. I will try to provide as much information as possible.  I took ~3000 interval photos (8256 x 5504) with my Nikon D850 and sucessfully imported the combined photos into the project pane as one video in adobe premiere pro. I then added the video to an empty sequence pane to create the sequence, and confirmed that the sequence settings are matching the source (8256 x 5504). I then added a soundtrack, a title and exported the video as a H.264 format file and made sure the export settngs also match the source (all screenshots attached). However, there are still dark blank lines on the side of the video. I am using a 4K Monitor with 4K resolution. The video details show that the resolution is 8256 x 5504, but if I upload to youtube it has dark black lines on either side. Viewing a video on youtube that is 8K is still fullscreen on my 4K monitor Any advice on why it's not full screen would be greatly appreciated.