This is not a bug, the separate values of the resource production bonuses are shown separately in the annex/colonization window.
Perhaps not a bug, but it did show them separately on the tooltip 2 months ago before the world map overhaul. In terms of the overall user experience, I think it was more clear to always show the random bonuses separately from what they might stack with. While I do enjoy math, I shouldn't have to do any in order to identify how much of a random production bonus a given tile has. Also, for the sake of consistency, I would argue that the tooltip and the annex dialog should not differ in the way that they show these bonuses.
Also consider the use case where I'm not actually the one doing the annexation, and it's another player asking me for advice. Or consider the case where I want to inspect such information before my next rank of centralization is ready. In both cases, I can't even invoke an annex window to begin with.