what?

I´ve been extending for 2 months nonstop
9 hours a day.

Seemingly i had achieved a NBP-FSL measured in the extender of 18.9

I did a rest day, substituting extender for couple aggressive hand stretching sessions.

Next day suddenly the NBP-FSL in the extender dropped around 0.4cm
Next day seems like slowly recovering the lost stretched flaccid length

Theories for why that happened?

Tissues that are in a “stretched” state will naturally return to their regular condition. So when you stop doing PE, there is always some shrinking.

But your exercises aren’t meant to just stretch the existing tissue. They’re meant to stimulate growth and remodeling of the tissues.

You said aggressive session. Sounds like you did more than you could handle and took a day to recover. Normal occurrence at times.