Not sure what device you are referring to, but in the case of the iPhone it is not tagged incorrectly, just a little stupidly.
If you have the volume buttons on the top, logical as they double as shutter control, the iPhone records video upside down, but then tags it as needing 180 degree rotation. No idea why they did it that way, but hey!
Apple devices, Adobe Photoshop and probably others successfully interpret those tags and rotate the video. Windows and MC do not.
It would be good if, at some point, MC was able to read and apply the rotation metadata. For now I use Photoshop to export the video to a new file, which fixes the problem.