(I know this post is like 3 months old but I thought I'd chime in anyway).
I think The Last Light of the Sun actually is the first time he incorporated that tense-shift into his writing.
I'm not entirely sure how I feel about it either. Present-tense is also used in Under Heaven and actually feels even more discordant... The usage in Last Light at least made sense to convey that a certain character perceived the world differently, but otherwise it feels somewhat incongruous to me.
It's not intolerable or anything (GGK remains my favourite author) but it's... odd.
