Hello,
I've encountered the following issue, playing as Yor:
Drengin Empire started a war with me and as I was pretty low on firepower, I bought 6 ships from the Iridium Corporation (for the measly sum of 900 credits, but that aside).
I could not move them after they came under my control (they all had 3/3 range), not even after setting them to sentry/guard/explore or even upgrading them to my own design.
I sadly had to decomission them.
I've been (superficially) been searching this forum for any post which describes the same issue, but I haven't found any. If this is a know issue, my apologies.
Kind regards.
[Edit]
Figured out it was due to the life support modules, the ships were out of range to begin with. This does render the option to buy ships (early at least) via Diplomacy rather useless.