I think we have cache coherency issues within a single draw call. One of the
situations in which a feedback loop is allowed is if we read and write every
texel at most once and we read the texel from the fragment shader invocation
that writes that texel.

My guess is that this is the case here, with two triangles. The problem is that
while every pixel is written at most once, every block of pixels can be
modified multiple times on the edges of a triangle.

This results in garbage if a later invocation for a tile sees modified pixel
data but stale DCC compression data, or stale pixel data and modified DCC
compression data.

This can be fixed by decompressing the framebuffer and disabling DCC
decompression for the framebuffer. I would like to avoid decompressing, but I
do not know how.