TAA sometimes creates splotches along the edges of certain blocks. Kind of looks like if you averaged the 2 colors and traced over the edge with a thin brush without anti-aliasing.
This doesn't occur (or at least not very noticeable) with most blocks and it happens regardless of settings or resource pack.
Mesa 25.2.6
RX 9060XT
Minecraft 1.21.8
Iris 1.9.6
Complementary r5.6.1