Had Byzantium lost the secret of Greek fire by 1350?

Was the secret of Greek fire already lost by the time of Winchelsea?

Largely yes; Byzantine turmoil progressively reduced the small circle that knew how to make and use it.

The technology depended on a narrow community of specialists. Repeated internal crises left fewer people able to transmit the method until the necessary knowledge was effectively gone. Its loss appears as cumulative institutional attrition rather than a single moment when the formula vanished. Greek fire therefore illustrates the fragility of secrets kept by very small technical communities.