The loop that runs the evaluation of constant expressions in the SBE engine continues running until modifications are made to the tree, but if all the unused variables have been removed, the loop is stopped.
In the commit for SERVER-101306 this early exit has been removed in order to re-run a folding operation on a newly created node, that needed an extra run after the tree was modified. This extra run has an impact on performance when constant folding a tree with a lot of variables, because the time spent resolving variables is significant.
The decision is to put back the early exit and find a different way of triggering the processing of the new node