As visible in the attached screenshot, a surgeon "repaired" (affecting only the core integrity) a broken "friendly"¹ derelict, which is kinda completely useless.
¹Marked as friendly by the [2 - Friendly] button, which isn't always the case for broken derelicts.
(Actually, the bug might be that these are marked as friendly(?), unlike what was mentioned here : https://www.gridsagegames.com/forums/index.php?topic=573.msg4918#msg4918 )
True, they're technically allowed to do that. Guess I'll block broken robots from their possible repair targets since it doesn't do much! (would be pretty neat if they could restore functionality instead, but don't want to imply that's possible in the game)