That is a tiny deviation from the torque spec, to the extent that it's probably within the bounds of the inaccuracy found within a typical cheap set of torque sticks. For the freedom eagles in the audience, a difference of 20 Nm is 177 inch pounds, or 14.75 foot pounds. In terms of, say, head gasket bolts that's rather a lot. In terms of lug nuts, it's basically nothing.

TL;DR: You're fine. And whatever your issue might have been was not caused by your lug nut torque.

I assume you drove your car after having the wheels re-balanced. Did that fix it? If so, there's your answer.

I don’t think so. I’ve seen lots of mechanics just slam the impact wrench on lug nuts instead of torquing. I’m not a mechanic tho. Only know a few cars I’ve worked on.

There's no way that slight over torquing would cause any problems like that. You could damage the lugs, but not at 140Nm.

Look into road force balancing.

The symptoms were likely tire imbalance, but its possible if the wheels were not mounted flat to the wheel hub and torqued down at an angle, it could also make the same symptoms.

20nm/15ftlbs isnt really that big of a margin of error for lug nuts, in my experience. I have seen guys use pneumatics only to tighten them, and while I would not do that myself, I haven't seen one come back for that kind of issue, and I know it was way more than 20nm difference. Overtightening lugnuts is far more likely to shear the lug stud off, if it doesnt crack or warp the rim or wheel hub.

If the tire imbalance wasn't the cause, its possible the wheel was mounted at a slight angle due to rust or debris on the hub, but if the symptoms went away then the wheels might be flat to the hub too.

Just so you know: all torque wrenches have built-in inaccuracy. All of them, even the really expensive ones. So your wrench might say 120Nm, but it might actually be 115Nm, or 125Nm, or maybe even more inaccurate. Most reputable tool manufacturers make their torque wrenches with +/- 4% accuracy, but that doesnt mean they all are like that, even from the good brands. Don't drop it and do not store it set with any tension in the tool, because those also can easily damage the tools accuracy. Generally, you should check calibration every year if you use it infrequently, and every 6 months or sooner if you constantly use it.

Not anything to worry about. It’s not the first time. Not the last time.

140 is quite a lot, but it isn't what cause the vibration in the first place. They do it that tight just to be sure, no one wanted the wheel fall off and people killed, a stripped bolt is still preferable consequence.

20g off the balance is quite a lot, 40g will likely cause vibration on 70-80kmph. Usually 15g off it will caused an issue on highspeed from my experience, but less than 5g usually won't cause any vibration for anything less than 150kmph..