Bug in maps with epsg codes?

I want to understand well how to use EPSG codes for projection. So I overlap two maps with the same projection (UTM 20S) with differents colors. In black I use the -JU-20/15c and for the red and green I use EPSG:32720/15c.

For the left figure I use the region AR.B+r6 and there is a perfect math between the tow maps.

But for the right figure I only modified the region (AR.B+r7) and there is little difference in the frame, annotations, gridlines and coastline (make zoom to the bottom left corner of the right figure).

Is this a bug? Maybe it is related to with the issue 6938. Or am I missing something?

gmt begin bug png
gmt set MAP_FRAME_PEN faint
#   OK
    gmt coast -Bafg2 -W1/faint     -JU-20/15c        -RAR.B+r6
    gmt coast -Bafg2 -W1/faint,red -JEPSG:32720/15c  --MAP_FRAME_PEN=red --MAP_GRID_PEN=green

#   WRONG. 
    gmt coast -Bafg2 -W1/faint      -JU-20/15c        -RAR.B+r7 -Xw+2c               # FAILED
    gmt coast -Bafg2 -W1/faint,red  -JEPSG:32720/15c  --MAP_FRAME_PEN=red --MAP_GRID_PEN=green
gmt end

I would say the right ones are also equal but for some reason the two are slightly offset vertically.

Yes, It seems that there is an offset of -Y0.055c.

gmt coast -Bafg2 -W1/faint,red  -JEPSG:32720/15c  --MAP_FRAME_PEN=red --MAP_GRID_PEN=green -Y0.055c