Is the map tier bonus from voidstones additive with the map bonuses from the tree?

I keep seeing people with conflicting opinions on this and I think that the fact that people think that they are multiplicative is just misinformation caused by the fact that the game doesn't add those numbers together within the map mods list. The game almost never takes mods from the atlas passive tree and puts them in the map mods list, with notable exceptions being the pack size wheel which actually shows up within a map.

I wanted to make this thread so that we finally have a source on this that we can link to whenever someone asks this question. So, to reiterate the question:

Is the map tier bonus from voidstones additive with the Shaping nodes and other small chance for map being of a higher tier nodes?

Additionally, beyond the question of voidstones being additive (or not) with the atlas passive tree nodes, it would be useful to understand the interaction when exceeding 100% chance. For instance, in Ruthless, with 4 voidstones (25% each) and a number of the small atlas passives at the top of the tree (2% each).