I am trying to grow a large oak tree using the "force" method described here: https://minecraft.gamepedia.com/Tree#Oak_tree

The method is:
A large tree can be forced to be grown if a "brace" of non-solid blocks (e.g. glass, or slabs) is placed around where the trunk would be, one block off the ground.

I did just that... glass blocks... 1 block off the ground (even on the ground 3x3x3 leaving the centre hollow made no difference) ... any new map in 1.16 Release Candidate 1.

Basically, the tree can sometimes be large, but more often is small... i.e. no branches (where the definition of "large" oak variant indicates there will be branches)

This is normal growth behaviour when there is no brace around the trunk ... so I can't "force" a large oak tree to grow :/