Things can happen after Rotation. On one of my phones, the sound in Videos is reduced. I’ve no idea why. One day, I may investigate it. But the point is, Rotation can do more than just rotate, presumably because of a bit of bad software code.
Welcome to Android Central! Rotating to landscape probably means increasing the resolution of the video. If your phone has weak specs, it might have more trouble pushing all those pixels. You could try reducing the resolution of the video in landscape to 360p or even lower to see if that helps.
I would pause whatever you're doing before you rotate the phone... I've had lower spec phones give me problems when I rotated them while something was playing, but they worked fine when I paused the video and rotated the phone, let it adjust to the new aspect ratio, then started the video again.