YM 16622 - hour indices discoloration?

[JBurk84]

Hello, I am currently shopping a few YM 16622 and one (circa 2007) that I have my eye on has some (IMO) odd discoloration on the hour indices. See attached.

As a side note, I've seen images of the watch in a darker room and indices in question are luminous however the lume at the 9 o'clock discoloration is still present in the dark.

This watch in particular has my attention because, I trust the seller, overall condition (except hour indices), with box/papers, warranty, etc. - my only question(s):

(1) is this discoloration normal or to be expected on any 16622?
(2) what could cause this?

Thanks - James