Actually if I’m not mistaken, movies that are pre-rendered, or “printed to 7.1.4 such as Disney will not scale up to larger layouts. For example even if you have wides,(9.1.2) they won’t be used, unless the mixer has a encoded an object to be there. Otherwise the system will phantom image between the front and side(or surround in a 5ch bed). The upside is that DTSnx and I think DSU will upmix to those positions.
This is similar to how 7.1.2 mixes like ready player one use the top locations. If you have .2 tops it will playback normally. If you have .4 the tops will all play at the same time with no front to back panning. And if you have .6 only the top middles will play. Again, the upmixers will change that. That’s my nutshell version, and certainly things are missing.