Comment by CM30

Unfortunately the answer is yes. If a video has a vertical resolution and is under 3 minutes (previously 1 minute), YouTube will automatically treat it as a short now. This basically means every video related to a DS or 3DS game, or recorded on a phone in general, is now treated like a short by their system.

This has likely screwed up all manner of videos, since a vertical resolution doesn't equate to it being designed for a TikTok or Instagram style feed...