Okay, as it turned out the behavior described below is standard.
After finishing the Quest and returning to the mine-town altar
the next killed Aleax again had the gear that I was wearing...
a - a long sword
b - a katana
c - a helm of telepathy [1AC]
d - a pair of gauntlets of power [1AC]
e - a pair of dwarvish boots [2AC]
f - a cloak of flight [1AC]
g - a Hawaiian shirt [0AC]
h - a gray dragon scale mail [9AC]
This time I had changed my boots to something else and the Aleax'
inventory reflected that.
This implementation might make some sense to create spare armor
in case of lacking MR and getting the own armor disintegrated
by magic (or by dragon ray disintegration). Moreover the armor
seem to be always +0 but also rustproof/fireproof. I made use
of that by changing my non-fixed water walking boots by the one
that the Aleax provided. (And I'll take the Hawaiian shirt with
me, of course, for my shirt collection.)
But what's the point of this feature?