If it's a case that healed men return to the unit with the same experience they had prior to getting knocked out, then I don't think it's a bug. If the healed men have less experience than they started with, then that might be a deliberate gameplay mechanic, or indeed an oversight. It might explain why in some very rare cases people have reported units losing experience after a battle even when the unit has been involved in very little fighting. It sounds like a lot of effort to fathom it all out to be honest.