Usually plywood shrinkage manifests itself at the plywood seams not hundreds of little cracks....a bit of a mystery.
This is one of those situations where it is difficult making an analysis without actually viewing the problem.
Some photos might help.
Wish I could make more suggestions.
_________________________
Bart Blainey, B.E.R.
Consultant/Journeyman Carpenter
Building Envelope Specialist
Master Renovator
provip@prodeck.org
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