Conversion to grayscale fails with artifacts
I was checking to see if GIMP-2.99 updated today had the same problem rescaling grayscale images as GIMP-2.10 as reported in this bug: #2626 (closed) .
Unfortunately in GIMP-2.99 the conversion from RGB to grayscale produced the following:
So in GIMP-2.99 it looks like converting to grayscale isn't working. The last commit in git log is 637105b9 updated and compiled just now, but the same problem happened when using code updated through Dec.7, 2018.