Seriously, what the fuck? I thought it was some kind of user interface glitch so I looked it up. Nope, they're A/B testing this shit.
Found this brilliant statement by Youtube on the matter:
Now note that we are deprecating one sorting option and that is by oldest video at the channel level. But don’t panic, you can still view the oldest video of a channel, by scrolling through it’s content’
Oh, yeah, just scroll to the bottom. Silly me, I guess I'm just a stupid idiot for not realizing the option was redundant since I can simply take the time to scroll through potentially thousands of videos.
Why are silicon valley tech people such freaks?
This is why I just stick to bookmarks or written lists. I stopped caring about favoriting videos in YouTube about a decade ago because the list ended up being half [removed video] due to DMCA claims.
If nothing else I wish they would keep the title of the video up or something so you'd at least know what it was later, instead of possibly forgetting it forever.