Upon further testing, found the reason and solution for the above mentioned issue. So now the media player has a
Zoom Mode with 3 options Full, Fill (these 2 seems to pretty much do the same thing) and Original size. For 4k resolution videos, to display in proper aspect ratio with black bars you have to select Original size. The irritating thing is that the application always defaults to Full instead of remembering your selection. So next time you run the app, you would have to select Original size again.
But this causes an even bigger problem when trying to play 1080p or lower resolution videos. Since the Original size option does what its name suggest, this happens
So in this case, you have to select Full or Fill to stretch the video onto full screen.
Idk why they made such a change to the media player with the new firmware since the previous version of the media player was able to flawlessly play and display pretty much every kind of video i threw at it. Tempted to feedback this to Hisense SG but i doubt anything would come out of it.