I see this from time to time and have no explanation as
to why it happens. I’ve worked around it by trying any
one or more of the following:
1. Fading up using keyframes on the opacity line of the clip
rather than the pre-built dissolve effect.
2. Creating a new sequence in 10-bit uncompressed, copying
the entire 8-bit sequence into it and rendering it all out 10-bit.
3. Bring the pict file into Photoshop and export it as a jpg or tiff.
Good luck.
– Michael Drucker