Portion of previous frame erroneously appearing over current frame

[beaswag]

I am processing a GIF file using gifdec. In a certain GIF I have made, when decoded using gifdec, the 3rd frame of the GIF erroneously shows a portion of the 2nd frame.

I have made a GIF which replicates this issue. The GIF is exported using Aseprite.

This is how the 3rd frame appears when decoded using gifdec. It is supposed to appear as a solid color.

It's likely that the incorrect behaviour is related to the GIF's frame optimization, it seems like the portion of the image which is showing up as the 2nd frame should actually be showing as the 1st frame, as that portion of the 1st frame is the same as that same portion in the 3rd frame.
This may be due to an incorrect implementation of frame disposal, but I don't know enough about the GIF format though to be able to say for certain that this is the cause.

I am accessing the image data using gif->frame, because my use case requires reading indexed image data from the GIF. If this is incorrect, please let me know.