Hello:
Well, there are a lot of things about the game that aren’t written down somewhere in game… and yes, there is no way of knowing about it unless you read about it “somewhere”. Some of those are bugs… some are quirks experienced by some but not all players, and others are actual game mechanics. Spending time on this forum is one way of learning about some of it. Now that you know about the restriction about the cube recipe for primals… I hope that you will take it in consideration before cubing a legendary item and upgrading it to primal.
There is a Game Guide but it hasn’t been updated for many years… and some of the info it contains is now outdated… and/or wrong.
There’s no way all that information about exceptions or restrictions could be fitted in the game… as others have said above, some of the regulars did try very, very hard to have Blizzard put in some info on the Cube recipe… because they expected the exact situation that we have now… to no avail.
So… in many more words that I wanted to write… that is what I meant.