Observe conservative insurgencies
The word observe is a bit odd, because the s is pronounced as a /z/:
Serve can be pronounced with /z/ when a vowel precedes, in reserve, deserve and preserve. But at the start of a word or when a consonant precedes, we generally get /s/:
serve, service, serving, servant, servile
conserve, conservative, conservation
For example, con/s/ervative:
So ob/z/erve is the odd one out (along with ob/z/ervation, ob/z/ervant, ob/z/ervatory, etc.)
Like ser-, sur- is also generally pronounced with /s/, as in insurgency:
And also absurd:
But just as ser- has a /z/ when preceded by a vowel in reserve, deserve and preserve, sur- likewise has /z/ when preceded by a vowel in usurp (which refers to the taking of some kind of power without proper authority):
At least that’s the traditional British pronunciation. From Americans, and many Brits today, you can hear usurp pronounced with /s/:
(There are some people who pronounce absurd with /z/; but I think the clips above represent the most common pronunciations.)
Surely you haven’t forgotten ab/z/orb and its relatives? I think it’s just voicing assimilation that happens to be lexically restricted rather than a general rule in English.
‘Manservant’ and ‘maidservant’ (besides being obsolete) are clear compounds, though written solid, and would never partake in this tendency; I would have omitted them from the list.