So I spent like a day looking into this for work and here is what I learned:
-You will never have seamless looping video in UE4.26 in preview mode
-You can only achieve seamless loops through a built game or a game running in standalone
-For this to work you need use a media source internal to the uproject (creating a file in the content folder called “Movies” putting your mp4 file directly in the uproject file from your file explore [not in ue4] and then creating the media source to that)
This is what worked for me using an H.264 codec at a Highbitrate with footage that was 15.4mb.