I’m familiar with the regional uses of that pronunciation, as well as the arguments for and against the idea that was how Hebrew was spoken in ancient Israel. I, for one, wonder if it an effect of the hellenization of Hebrew (as is the case with the translated forms of many Hebrew names) but I can’t claim to be knowledgeable.
I’m interested in finding out if anyone knows the history of the naming convention in English translations of Jewish organizations and titles. For example, Agudath Israel uses a -th sound even if most of its members use a sav sound or how the transliterated title on a copy of שמירת שבת כהלכתה will be Shmirath Shabbath. Just something I’ve wondered and google isn’t pulling through.