There are infinitely many test scenarios, and that is why we highly encourage to try or test a few scenes/scenarios, as close as possible to production (with a backup of course), so such things are found and tackled.
No one would expect such fundamental changes when reading "Added support for ForestPack Ivy Generator". In fact, many would not even have the newest FPP installed, so they won't care. It's entirely ok to change things/try out something but it happened in major version 12 as an "update"? (for whatever reason this was chosen - first time in Corona history). In a major versions "update" I do not expect such changes at all.
Basically it's very cool that the feature got support, otherwise the yelling would have started later anyway. But it is definitely a very basic change. It does not only affect lightMix and masks, it also affects VFB raycast picking. Now it is not possible any more to quickly pick that nasty, wrong looking FPP custom object to fix it. Instead, you get the FPP object itself.
By the way, that discussion is there for a
long time and as mentioned previously, users adopted the way Corona handled FPP and took opportunity from it because there has been no movement or commitment. So scenes use the "old" behavior, also v12 ones. Now that update in the same major versioning. I'm still not sure if it was an accident and some unknown side effect because of the last minute changes or a known implication. Basically there should be more time to test for users and developers anyway. Usually you are swamped with work and occasions to test are rather rare. But I'm repeating myself again.
In my honest opinion, this is not a harsh change, and even the desired behavior (selecting a single object and having all its instances inside other objects) is weird for me.
You can say the same about how hierarchies are handled. Isn't it weird that you get multiple objects in mask when including a single one? Yes it seems. But here the same: users adopted that behavior and link objects to another to have them semi automatically included in a mask. You would break again many scenes when changing this. As said: all ok in another major Corona version where users can expect fundamental changes - and if it gets announced.
I hope you get my point - I got yours. Meanwhile, as of FPP 9, edge mode in IR seems to be broken which is worse :)
Good Luck