I know if someone revives you with the Krauss Defibrillator it won't work that revive, but later it will. I've also heard after 4 downs you lose it. Correct me if I'm wrong though, I've only ever lost it after inactivity.
Nope that's false man I've been downed so many times on my bad days and it still works. and you've lost it I stopped playing for a month and I still had it.
I believe you can lose perma-tombstone on maps like die rise and buried when you pick up a free perk bottle after you already have all the perks.
I Too have lost it multiple times after picking up perk bottles on buried after I already had all perks
Well, I'm pretty sure it's the same for Die Rise and Buried, but I just did a test run on Die Rise and kept my perks. You get the laugh when you pick up the perk bottle which had me convinced I was wrong, but when I revive myself, I still have perma-tombstone.
I was unsure wether it was the reason for the loss, but good to know it's not for sure. Surprising we don't know definitively yet
According to COD wikia, being revived by the Krauss Defibrillator will cause you to lose all your perks in that instance, but not the persistent upgrade. Taking 3 consecutive downs without having any perks will cause you to lose the perma-perk.