I have not seen this before in willow oak.  I take a walk daily through a subdivision that is about 5 years old and there I have noticed that several trees, mostly maple, look similar to yours.  Some of those seem to leaf out much later than most trees and others never do.  Looking at the small branch in the first photo, I see what look like small buds along the length of the branch.  Those might be leaf buds, so I would give the tree a couple of more months before making a decision about it.  There has been some evidence, but no research yet, that herbicides applied to kill broad-leaved weeds in grass may be taken up by tree roots close to the surface and affect the tree.  It can also take a few years for a tree that was planted with roots circling the container it came in to cause damage to the tree.  You might also want to pull the mulch about 6 inches back from the tree trunk and reduce the mulch height to 2-4 inches around the tree.  This will better allow water to reach the roots and keep voles away from the tree trunk.