On Sunday, November 5, 2017 at 2:23:24 PM UTC-5, wrote:
...I heard a story/rumor/speculation that the fabric tape used to cover
the flap/wing joint might be to blame if it bulged out into the airflow
during slow flight and triggered separation.

IIRC that tape bulging caused aileron flutter, but not other problems.
Regardless, the tape should have been replaced with mylar on all 20's.
Hank???