Twice now I have sent Harness out to explore with a load of stimpaks and radaways however after a while I will come back and he will be dead so I need to use yet another 1000caps to revive him, however this time I looked at him to see what he had and he was slightly damaged and only slightly irradiated with 13 stimpaks and 10 radaways left. I clicked off him and went back to him only a few minutes later only to see him dead due to his health bar being fully irradiated with no stimpaks or radaways used.

I proceeded to revive him and recall him in hopes of not losing him but when I have revived him he has come back to life with his health bar still fully irradiated and isn't using stimpaks or radaways, also his character is showing the running animation but his face is showing as dead (eyes shown as crosses and tongue sticking out). I have a horrible feeling as soon as I allow him into the vault when he is back in 5 hours he will instantly die again

Desperately need a solution to this problem!

Well, apparently a returning dweller can't take any damage or rads, so just see what happens. If he wanders into the vault and then drops dead, I'll be really surprised. Never seen this bug before, and I hope it was just a one-timer for you too!