[OJFord]

What's the reason for capitalising either of them introducing any debate in the first place?

I doubt this will pass the test of time, makes it seem like a noun ('a White'; it must be a noun to be a proper noun after all) rather than an adjective ('a black person') which (capitalisation aside) is way out of favour. (At least it is/would be in the UK, where 'PoC' isn't really used either, so ymmv.)

[SoftTalker]

Agree. If you're hung up on whether your adjectives are capitalized, you don't have much to worry about.