Actually, I performed it on the whole image. On a crop, the colour is fine but the pattern is still there.
More interestingly, in CLI, the discoloration is different. I have only tried it once for each case (GUI full, GUI crop and CLI full) and with my quickly written filter. Have already spent too much time on this. Please verify whether the discoloration is consistent in each case and across cases.
PS Anyway, this could be due to a number of reasons: the filter’s age, FFT shenanigans, parallel processing, plugin vs gimp vs CLI speak, CPU/GPU drivers, compatibility issues, you name it… The good thing is that I can reproduce the bug to a certain extent.