Thanks, and good catch! I found it - literally one tickbox I mis-ticked. Got a fresh build with the fix up now, a fresh mode to go with it!
I think I extinguished at least 5 suns, and was unable to get the achievement. Probably had 20 suns all in one corner too, hoping to see something else created. First I had dragged suns into the storms, and then I tried to create new storms to place around the suns. Both made them disappear multiple times, but no achieve. \_(o.o)_/
I think I phrased the achievement poorly. It awards for erasing *all* suns, so if you still had more in the corner it won't fire. I'll look at either fixing the wording, or adding a new achievement type that triggers on individual removal.