Apparently the level I played on had an imprisoned neutral Vampire that went feral after loading a savegame in the same session. I found him running around wildly, was unable to contact him to recruit or cast Thunderbolt on him, but could blast him by Shockwave. Even after capturing both prisons he is still imprisoned and there is no way to either get rid of or convert him.
Nano Boucher please reimport the map "Stonegate" now after the large L-shape room freeze is fixed,
because prison walkabilities are wrong (units can pass and are barely blocked by the fence) and torture chamber props are rotated oddly.
Thank you
See for walkabilities, this is a correct prison: http://imgur.com/eW1xhL0
And this is the prison on imported Stonegate: http://imgur.com/97hOJwd
Yea the issue is map specific (caused by an importing error) and not related to save and load.
The issues allowed prisoners to escape the prison while still being logically imprisoned,
the vampire was already your prisoner because you owned the prison which allowed to pick him up.
Grabing and droping a prisoner outside the prison frees him / removes imprisonment which allowed him to convert to your side instantly.
I can validate that this is happening on the Public Linux version as well.
I attached my save file where this is happening.
stonegate_netural_vampire.tar.xz