I'm seeing the same thing here.
It's probably a bug... I submitted it to Adobe. One workaround I found is to create a new video track under all the rest and fill it with black video that is the length of the timeline. This causes the unsharp mask to render correctly during fades.
That's not surprising. I've also found that black under a title will cause it to be rendered differently using GPU acceleration.
Looks like a bug. Thanks. I'll escalate it.
Workaround looks good, so thanks for that too! Nice find.
LensFusion wrote: If the problem had to do with the Film Dissolve processing in a linear color space, then I would expect similar results with the Sharpen filter. Likewise, if the problem had to do with the Unshark Mask, then I would expect to see the same thing when it is applied to the clip directly.
I think it has to do with blend modes, but can't say for sure.