"The composition of Greek fire remains a matter of speculation and debate..." - Greek Fire's Wikipedia.
How did this come about? Wikipedia describes the Byzantine Greek version as a flamethrower. I see that as an effective weapon. Wouldn't this usefulness have kept its contents known to humanity?
This comes up regularly, and, yes, it's a matter for conjecture- we simply do not know how it may have been lost . I posted one set of conjectures here a few months ago, there was recently another posting by u/quiaudetvincet here with another source referenced. And it's possible someone else now has something more to say here.