I had the same issue in 3 kegs 2 weeks ago.
One was an Imperial Stout 2 years old. It quit at 1.030 or so. I set it out at basement temp and popped the relief to de-carbonate it. Then pitched a .5 liter US-05 starter. Finished at 1.020
The other 2 kegs were the same deal as you. Fermented 10 gallons in a Sanke, and transferred to cornies before checking the gravity.
I did the same thing - Popped the relief, pitched US-05 starter, and let them set for another week and a half.
One finished at 1.020 and the other at 1.018
Not too bad.
As long as you have fermentation happening, you'll have positive pressure inside the keg, and no o2 should enter through the relief valve.