I can't explain why my cruise control works differently than your experience; it just does!
I actually think your car is fine; I suspect you may be missing some of the nuance of what we're describing. The issue that tesla has acknowledged as a bug is fairly complicated, and I don't think it's been stated clearly in this thread.
So - assuming you've got the speed warning stuff turned off, no offsets set, all that, and have a EAP equipped car. And you're driving down a road, and the speed limit is 45 mph. And the car also thinks this, and the little speed limit sign is displayed to the right of the cruise control speed as 45 MPH (this is important - if there's no speed limit sign displayed, it'll work normally). If you are driving 35, and engage CC, the car will set the cruising speed at 45 mph, not the 35 you are driving at, and your car will start to accelerate to 45 mph.
Try this - and make sure you've pushed all the buttons in the above paragraph. Of course it doesn't have to be 45 mph; anything that's faster than the speed you are driving will work. If your car still stays at the speed you're driving, then all hail your car! I want mine to work that way. Mine doesn't.
As I mentioned, after a lot of experimentation with this, I sent a lengthy bug report about it to tesla support, and their initial response was that no, it doesn't work that way, it engages at the speed you're driving just like every other car. I replied back - "have you actually tried this in a model 3, and not the model S you're probably driving?". They then sent me a fairly apologetic response, explaining that the original respondent wasn't the right person who should have responded, and that yes, this is a known bug, and that yes, it will be fixed eventually. That was some 3 months ago, I'm surprised a fart app got precedent.