@jdonalds, I am not an EE (and don’t even play one on TV) but I’ll spitball here and throw out some WAGs.
Logically, the problem is either in your EVSE, your car, or the grid. And by grid, I mean anywhere between your JB EVSE and transformer supplying your house. So how do you narrow it down to which one of the three is the culprit?
Power line fluctuations could well be the problem but then why does it always happen towards the end of your charging cycle and not randomly over one cycle and between cycles? That observation tends to make me look at the JuiceBox. Could it be doing something anomalous towards the end of the charge cycle like hearing up enough to make a component fail?
Can you program the JB to charge at a much lower rate to see if the spikes go away? That might help determine if the problem is in the JB.
Is your JB the plug in kind and not hard wired? If so, if you can find a neighbor or RV park with the right receptacle, then you could try charging with it away from home. That might rule out grid problems at your house.
Even better would be if you could get a different JB to plug in at your house and charge the Clarity. That would really diagnose things but may not be practical. Perhaps you can find a local EV group to ask around.
And finally, I’ll suggest what would be a unicorn hunt in my area but might work for you. Can you find someone close by with a Clarity that would be willing to charge at your house? Then if there are no spikes that might narrow it down to your particular Clarity.
I hope you find the source of your problem. I know problems like this can be quite vexing because the larger the number of variables, the harder it is to solve the equation.
PS: I don’t think your external grid is the problem since the spike events are not random, but then I admit I don’t know squat about electricity. So maybe your should contact your POCO and ask them to check your incoming power. We had a problem at my last house where whenever several neighbors’ AC units started up at the same time that we would get lights briefly dimming. Our POCO investigated and upgraded the transformer we shared. They said it was getting old and to prevent reoccurrence, they installed a larger one.