Spigot 1.13.2 latest
EssentialsX 2.15.0.55-b656

Generally, bigtree will not spawn a tree if the ground is not dirt/grass. However

bigtree jungle

(for example) will spawn a 2x2 trunk even if only two of the four ground blocks are dirt/grass, replacing the other two ground blocks with dirt.

Prepare ground

Stand on closest dirt block and

/bigtree jungle

Chop tree to view ground

A check seems reasonable, but I was using bigtree only to test a tree-chopping plugin, so it doesn't matter much to me. I don't allow bigtree in production.

Tree generation is performed by vanilla/CraftBukkit, not by EssentialsX, so the only solution on our side is to add a manual check.

Considering this is a building tool, adding a check seems unnecessary.