Finding the words to describe this particular problem are hard, so let me explain.
I'm working on a complex 2D animation in Premiere, animating mask paths in order to create the illusion of 3D movement. The best way to explain what I'm doing is to show it:
So essentially, I'm changing the mask shape of a color matte, using lots of different layers. The issue here is this: as far as I can tell, I can change the scale without changing the shape of a mask in two ways:
Option 1. Manually move each point of the mask hoping that my eyeballing the shape is accurate.
Option 2. Animating Mask Expansion, which uses its own keyframes.
Here is where this issue starts. Using Option 2, if I make changes to the mask expansion when I have already keyed a change to the mask path, the animation, instead of smoothly expanding (from expansion key 1 to expansion key 2) and changing shape (from path key 1 to path key 2), instead smoothly expands without changing shape at all, and then immediately after the second key of the Mask Path jumps to the changed shape. This error is then locked to the particular mask permanently - if I delete both the expansion and path keys entirely and instead take Option 1 and manually move each point to increase shape, the animation will still glitch and jump from one shape to the other, even if I haven't used any expansion keys.
Like I said, it's complicated and hard to explain. It's a nuisance at best, and at worst can cause irrecoverable damage to your project.
Here's an example to illustrate this issue:
I've got these two animations in different clips, and I want to connect them together so that there is a fluid transition from one pose to the next. Call the top picture Pose 1 and the bottom Pose 2.
I copy the mask path shape from the start of Pose 2 and paste in on a key at the end of the Pose 1. The key's mask shape copies and pastes just fine and the animation transitions smoothly from Key 1 to Key 2. However, whoops, I realize that the Mask Expansion was set at -9 for Pose 2, while Pose 1 is at 0, so the animation is just a teensy bit off. I key in the Mask Expansion at the end of Pose 1. It should transition smoothly from Pose 1 to Pose 2 now, but instead it plays like this:
The Mask Expansion key transitions smoothly, but the Mask Path is glitched. Here is the actual mask shape and the keys:
Key 1 - everything is fine.
Key 2 - the Mask Path should be where the lines are, but you can see that it isn't.
The next frame immediately following Key 2 - the Mask Path is where it should be.
Deleting the Mask Expansion keys doesn't fix the problem.
Copying and pasting the entire clip and trying again without the Mask Expansion doesn't fix the issue either, it's somehow tied to that clip (Pose 1). I've tried keying the frame immediately following Key 2, where the path should be, and then deleting Key 2 - the issue persists. Copying and pasting the mask key from Pose 2 into Pose 1 (or keying the pose that was copied over from Pose 2 later in the clip) will now always cause this issue. And it's not just that key either, now copying any key from Pose 2 into Pose 1 will cause this issue. It's hard to explain, but basically any transition from a key copied from Pose 2 will do this glitch when preceding or following a key from Pose 1. A demonstration:
Key 1 - Original key from Pose 1
Key 2 - Copied key from Pose 2
Glitched mask appears without transition the frame after the second key and the frame before the third key
Key 3 - Original key from Pose 1
This section animated - there are no smooth transitions between Keys 1, 2, and 3.
I don't know how deep the issue goes, but what it means is that there is no way for me to get that shape mask from Pose 2 into Pose 1, and therefore I cannot ever transition smoothly between those two poses.
I hope the issue makes sense now, I know I got a little tied up in poses and keys and whatever but I hope I made it easy enough to follow. It's really hard to describe the issue, but at the same time it is really impacting my ability to animate for the project and I need to figure out if there's a solution to this or at least report it as a bug so people can maybe fix it.
In case anyone else comes across this and is having the same issues - I got in contact with Adobe about this bug, and after some generic troubleshooting I finally got to explain the issue and after a long wait the person got back to me and said that they don't recommend using Premiere for this type of work, and they couldn't help me.
So, unfortunately, if you get this glitch, according to Adobe, you are SoL.