Wrong disposal on first frame of transparent gif

[RedError]

Summary of the bug:
When creating a transparent gif with ffmpeg, the first frame has a wrong "Gif Disposal" value ("Do Not Dispose" instead of "Restore to Background") and thus is always overlayed on the second frame.

How to reproduce:

ffmpeg -i "slowsmoke.gif" -gifflags 0 -vf "[0:v] split [a][b]; [a] palettegen [p]; [b][p] paletteuse [r]" "slowsmoke ffmpeg.gif" -y

Input:

Output:

ffmpeg version git-2020-03-24-e5d25d1
Downloaded from ​https://ffmpeg.zeranoe.com/builds/

Transparent gif "slowsmoke.gif" provided in attachment, also available here: ​https://i.ibb.co/mzSx63y/slowsmoke.gif

Additional details:

• Gimp shows the second frame as being "combine". Editing that information allows the gif to be fixed. This proves that it is indeed the frame disposal that is in cause.

• -gifflags +offsetting leaves additional traces (by reducing the redraw area), so it's better disabled with -gifflags 0 like I did, but that doesn't actually fix the underlying first frame disposal issue.

• -gifflags -transdiff or +transdiff doesn't actually do anything for transparent gifs.

Attached:
Report file from ffmpeg
Input image (transparent gif with movement between the first two frames)

[RedError]

Input file (transparent gif with movement between the first two frames)

[RedError]

ffmpeg report file (-v 9 -loglevel 99 -report)

[Carl Eugen Hoyos]

Is this really different from ticket #7902?

[RedError]

Replying to cehoyos:

Is this really different from ticket #7902?

No, you are right.
I had searched for gif disposal because that was the actual problem but it is about the same issue, just explained with less precise terms in #7902.

[Carl Eugen Hoyos]

Please feel free to add any missing information to ticket #7902.