I recall reading somewhere that it's an artifact when infusing from an item with a light level ending in 1. (371, 381, 391)
The first Youtube video I found supports this theory. Unfortunately it's unlikely that infusing your 399 chest into others will give you that last bit of light you need.
Now that you make me think about it, the couple of times it happened to me was when i infused an item with a light level ending in 1...
Well, thanks for the info, saved me from wasting time and materials. Let's just hope the raid is going to be more generous this week ^^