The obvious explanation would be bass reflex.
The paradox: Group delay remains close to the sealed enclosure.

Red = sealed.
Green = Dynamic Zero.

The low-frequency gain is there:
about +10 dB broadband below Fs.

The expected port-delay signature largely isn't.
This does not prove any particular mechanism.

It simply raises a question: What is the system actually doing?

Build it.
Test it.
Refute it.

GitHub: https://github.com/DeltaSignum/Dynamic-Zero