Thought i'd pitch in that i found the cause for this, posting it around the web.
Long story short, in various regions youtube are capping the resolutions available to devices, especially if they're not sold in that region. This may be accidental. This bug is most common for people who import their phone, or run custom roms.
In my case, i had a G900i and N910G (S5 and note 5) and their respective custom roms changed the model numbers over to G900F and N910F respectively. Both suddenly lost all youtube resolutions above 720p, and the new 360 degree videos on youtube would not work.
using a root file editor and changing system/build.prop allows you to edit the model number your phone reports itself to youtube with - changing those phones back to their correct model numbers or even SM-N9005 (the model of Note 3 sold here down under) and *rebooting the phone* suddenly unlocked youtube to 1080p on the S5, and 1440p on the note 4 (even with the Note 3 model number, which i found odd) and the 360 degree videos suddenly worked.
If you're confused that your phone isn't modded or imported and still suffering this bug, it just means your ISP/telco has connected you to a youtube server that must be located further away than you'd think. changing your DNS servers or using a VPN would be alternatives, although more annoying.