Thanks for the comments. In the PB editor the lava displays correctly but when the sandbox is run you see the rendering glitches. I've tried many time with different combinations of tiles but so far not been successful. If you have a way to remove the glitch please let me know.
no problem. it's annoying when the editor shows something else than when you actually play it, especially water and lava.
the ground tile was on wrong side (it should be a hard edge from the lava side, not the other way around). that won't work for you because you have only one space for ground. there is no variation of one-wide ground tile without grass.
if you can't really fix it, just leave it. as I said, it was just a nitpick and not a big deal.
Did you manage to complete the sandbox Gizgord. If so I can mark you down as the first player to do so !
yup.
it took me like 15 minutes, but I was taking my time.