I haven't investigated this at all but I've seen it often enough to be sure it's a bug in the stock game.
When respawning is enabled, kerbals are supposed to be set to "missing" and will respawn after some time. There is some pathway in the game that will sometimes set them to "dead" instead, which prevents the respawn. This doesn't seem intentional.
Rather than trying to fix that code pathway, we could make a patch that sets any dead kerbal to missing on game load (when respawning is enabled, of course) - that will also have the nice side effect of fixing already broken saves, or support a scenario where you turn on respawning mid-playthrough.
I haven't investigated this at all but I've seen it often enough to be sure it's a bug in the stock game.
When respawning is enabled, kerbals are supposed to be set to "missing" and will respawn after some time. There is some pathway in the game that will sometimes set them to "dead" instead, which prevents the respawn. This doesn't seem intentional.
Rather than trying to fix that code pathway, we could make a patch that sets any dead kerbal to missing on game load (when respawning is enabled, of course) - that will also have the nice side effect of fixing already broken saves, or support a scenario where you turn on respawning mid-playthrough.