Dear Ryo,
Surprisingly I also got a similar result from it as you mentioned.
I also found that the graticule line is missing in every plot when changing the second latitude point. This missing graticule line started from 17°N when we fixed the first latitude point at 33°N.
yes? shade/x=100:150/y=17:33/L=1/grat=(dash,color=7) SST
Best regards
pratik