So, whenever someone (or me) puts Fortnite, fortnite, or FoRtNiTe in their project tags, it turns into FORTNITE.

My browser / operating system: Windows NT 10.0, Chrome 68.0.3440.106, Flash 30.0 (release 0)

I'm not sure, but I think it's because there's no differences between capital and normal letters, than scratch change the tag to the most common one. Try with other tags maybe …

Maybe you should try these instead:

Fort-nite
Fort nite
-Fortnite-

Weird, maybe they made it like that on purpose because they know people are using it. Still odd to specifically change the capitalization of FORTNITE

Maybe it's because that's how the tags tend to be capitalized. I remember when I used to add tags to my projects, I'd occasionally get a tag's capitalization switched.

it normally turns all captial into lowercase.

I get things similar to this sometimes. Once I typed ‘cuphead’ into a tag, and it changed it to ‘CupHead’.

Example:
560 projects have “FORTNITE”
340 projects have “fortnite”
And 100 projects have other ones

But “FORTNITE” has the most so it will correct as if it is a spelling mistake.