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.

Try forcing highp precision everywhere in the fragment shader, disabling mipmaps, using nearest-neighbor filtering, and verifying UV alignment. Compare encoded/decoded values in compute vs fragment shaders. Also check for texel padding and interpolation errors at probe boundaries.