Why razing a ship did not remove its strength deck

Did removing the upper deck to razee a ship of the line make the hull structurally weak?

Not necessarily; the surviving gun deck remains a strength deck and the reduced structure is still proportionately supported.

The gun deck remains a strength deck after a two-decker is razed. It becomes the exposed upper deck while retaining framing proportionate to the structure it supports. Gun decks already have to carry the weight of the battery and withstand firing loads. Removing the upper deck therefore reduces the ship's weight while preserving the main support for the remaining structure.