1080x1080 files converting from mp4 to gif as 538x538

I downloaded my 1080x1080 file as an mp4 (Video Resolution: Custom (original size) then used Adobe Express's Convert to Gif page to convert it to a gif. The final step in my process is to open the gif in Photoshop so I can limit the gif loops using Save for Web.

I've done this exact thing with dozens of files and have never had an issue with my gifs not being the exact size I build them in Adobe Express. This time my 1080x1080 file is opening in Photoshop as 538x538. 

I decided to rebuild the file in Adobe Express to see if something was wrong with the file I was using but the exact same thing happened.