Sometimes remuxing the video into a different container, or even back into the same container, will fix issues like this. probably some tiny little glitch when the video was first created, and the remux cleaned it up. Glad it worked for you.
Dan
Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.