I have a feeling that this is unavoidable but I’d like to raise the issue for comment since the results are not intuitive when you see it happen.
Here is a screenshot showing a simple short animation of a blue rectangle moving to the right across the canvas. The lightbulb is turned on so the onion skins are showing:
Leaving frame-1 as the selected frame, then putting a transform mask on it with a small amount of skew and a small amount of rotation, the result is shown here:
For each frame, the pivot point of the rotation is at the canvas location that it was at when the transform was made on frame-1.
So, instead of a skewed and rotated rectangle moving to the right, the skewed and rotated rectangle moves to the right and up.
This happens for all versions including the May 06 5.0.0-prealpha.
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Yes, I explicitely remember @emmetpdx and @eoinoneill trying to figure out how to handle this. I seem to recall it’s a limitation of how rotation is implemented.
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I had a feeling that it was a limitation of some kind. Now that I know the developers are aware of this, I’ll stop worrying about it 
@AhabGreybeard @wolthera Sort of yes and no with regards to support for this ability. Rotation origin can be changed, but you need to enable it using the Transform around pivot point (alt) button in the tool options docker / window. The pivot point marker can then be dragged to mark the relative origin point of the layer.
I’m not entirely satisfied with how it works, but it’s technically doable. There are caviats though, such as not being able to change the origin mid animation (origin is fixed, meaning old rotation data “meaning” will be changed.) This isn’t completely unlike blender, but there are a lot of ways around it due to the (rather complicated) options you have as an animator with parenting in 3D software.
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I have spoke of the need of emptys before and parenting this to it and animating it in parallel would probably solve this.
