Why do the Japanese pronounce dozen “dasu”?

Losing a final n is very uncommon when English words are borrowed into Japanese, and this doesn’t seem to be a borrowing from another language, so it’s always been a puzzle to me.

According to English in Japanese by Akira Miura,¬†the pronunciation¬†might have been taken from the written abbreviation “doz”.

A whole article of mine on Japlish pronunciation here:

Pronunciation changes in Japanese English