Ok i went briefly into my SP game to check the issue, and eventhough i was thinking i was dumb to go and check that on SP (since SP = systematic server restart after every log out) it appeared it was somehow not so dumed after all…
-I logged in near sepermeru where all my thralls were at FULL health ( this is the last base i built in my current game )
-then i traveled to mounts of the deads (more precisely right under the ymir teacher icon) where all my testing thralls were at FULL health as well ( this is no true base, only a maproom and some workstations and thralls were placed here after my base from the black keep was finished )
-then i went to the swungle to find all my thralls at LOW health (this was my second base built on the map)
-then i went to the black galleon, all my thralls were at LOW health ( this was my first base built on the map )
-And finally i visited my base near the black keep and all guards were at FULL health (this was my third base built on the map)
Only pattern i was able to find here is the chronlogical order in which thralls were placed on the map, that’s why i wrote when each base was built at the end of every result.
So it appears that all my most recent bases had thralls at full health and all the oldest bases had thralls at low health.
i will try to do the same checkings the other way around tomorrow to see if sthing varies.