The label on the image should be 2x2 quad seam.

It is worth noting this artifact only appears when using a fragment shader. Doing the sampling in compute does not exhibit this artifact.

encoding & decoding code is same as presented here: https://www.shadertoy.com/view/cljGD1

but trying different implementations changes nothing. Also I was not able to find any NaNs in any of the code. It is also independent of the texture used or it's properties and format.