Not to mention St John becoming Sinngen, and Belvoir becoming Beaver.
I won’t pretend enough authority to say that it answers the OP’s question, but this topic should not miss mentioning the Great Vowel Shift.
Wessex is still occasionally used as a sort of nickname for Somerset, Dorset, or the combination of the two. Sometimes the whole West Country, as well. When I was at school in Dorset most of the interscholastic county sports were styled as the “Wessex Under-18 Tiddlywinks Cup”, or whatever.
Or the even more extreme transformation of Featherstonhaugh to “Fanshaw”. At least those first two may preserve the number of syllables from the way you might naturally pronounce the spelling (depending on whether you think “Belvoir” is 2 or 3).
And how come the Earl of Sandwich doesn’t live in Middlesex …
Sound change of vowels can be influenced by nearby vowels. So it’s possible that *Wessax and *Essax assimilated into Wessex and Essex. And then *Sussax and *Middlesax probably changed to Sussex and Middlesex because everyone thought they should be similar to the others.