I've seen this once before, on a pushrod V8. I don't know why it wouldn't be plausible here as well. That motor had a broken valve spring, so when the motor was cranked, no compression. When air was introduced at 100psi, it was enough to slap the valve up into place and hold it there. I surmised then that this is one reason why a leakdown tester has a valve that allows one to introduce air more gradually.
If I were you, I'd get a borescope in there and have a look around.
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