Operating system: Windows-10-10.0.19041-SP0 64 Bits
Graphics card: NVIDIA GeForce RTX 3090/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 466.11
Broken: version: 2.93.0 Beta, branch: master, commit date: 2021-04-28 22:25, hash: rB48bbeaf38304
Short description of error
If an object has motion blur between a holdout object and the camera, transparency of the motion blur is affected. In addition, this creates artifacts in the RGB channels as well (although this may be the result of pre-multiplied alpha). This is the setup created by motion tracking, and makes it almost impossible to get reasonable results for compositing with "hairy" objects.
The effect seems to be that the transparency falloff is much sharper than the areas without holdout objects behind them, but this appears to also cause artifacts in the RGB channel. You can see this effect clearly if the alpha is stripped from the image:
It seems likely that this behavior has been in Blender for some time, so it is possible that I completely misunderstand the operation of holdout objects. If so, I apologize. But given these are used in precisely this way for motion tracking, and I chased these artifacts for several days back to the cause, it seems like a problem that should be looked into.
Exact steps for others to reproduce the error
Render frame 5 of "hair alpha test.blend" and observe the output. You can see the outline of the "top" of the holdout plane, even though it is fully behind the moving hairball at that point. Since it is hair, this effect is very visible, but it is detectable even for "hard-edge" objects. If you save the resulting render as an RGBA .png, and remove the alpha, you can see bright artifacts in the RGB channels. When compositing, these artifacts can easily "leak" through the alpha (depending on the app/operation applied, such as blur), causing visual weirdness beyond the incorrect alpha channel.