Some variation is normal between the temperatures of different cores is quite normal, but 12C seems very large.
The first step I would take is the same as you already plan; reseat the CPU cooler with a fresh TIM installation.
If that does not fix the variation, it is probably a buggy temp sensor in the core and there is not much you could do about it.
Still, 86C in prime95 is still quite good!
I would not be worried about your temps at all at this point.
Not only is this within spec, it is also hotter than you will likely ever hit under normal usage.