The antenna is effectively a short circuit on 80m, by design.
When using only the center pin of the coax, you're using all of the coax plus all of the antenna as a long wire antenna. As soon as you connect the outer conductor (connector shell), that "feature" goes away and the antenna is essentially a short circuit on the end of the coax.
It was never designed to be used on 80m. Actually it was never designed to be used anywhere except 25-30 MHz.
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