What he said^^^^^^^^^^^^^^^. That amount of resistance change is totally within the realm of normal. I find the invoke kit to be excellent, as well as many of my other mods who occasionally show a resistance change. It's just... No mod reads everything perfectly, and even something as simple as stepping outside from a warm room into the cold could change the resistance by 0.01. It's not worth worrying about. If you think the 510 connection is loose, that is another matter, and perhaps worthy of a warranty. But not for the resistance change alone. The invoke is one of my favorite mods, and many others really like it. You can warranty if you think it's a genuine concern, that is up to you. But no mod is going to read resistance perfectly all the time, or at least most won't.
Best of luck, I think the Invoke is a fantastic device, and may be working fine, but you are the one who has to vape it, so if you aren't satisfied, all I can say is to warranty the device, perhaps the replacement will be exactly the same, perhaps it will improve.
Best of luck with whatever you decide...
Anna