Normally, in the overworld all air blocks below y ~10 are replaced with lava blocks. In terrain (even default terrain) generated by OTG this doesn't happen.

This is using:

• Spigot: 1.12.2-b1442
• OTG: v1.12.2 v1

Hey there, this is intentional and not a bug. We changed the <10y lava layer, it has been replaced by lava lakes. Thanks for reporting though :).