Car temp gauge spikes while idle, drops to normal while driving. What could be wrong?

For reference, I have a 2009 Nissan cube, 175,000 miles, so an ancient being. It recently had a coolant leak, though I suspected just at the reservoir since the reservoir would be empty in a day but the car never overheated, hovered around 6-7 ticks on the gauge when operating which seemed normal. Recently got tired of adding coolant every week just to make sure it didn’t run low and used some stuff from autozone that was supposed to seal leaks in car coolant systems, figured I’d give it a shot.

Fast forward a week later, reservoir still sits at max but the car has recently started to spike on the temp gauge while idling (about 10-11 ticks on the gauge) then settle back to 6-7 ticks once I start driving. No codes popped up on my OBD reader, the radiator fan still looks to be spinning (though I have no real basis for what is normal rpms for the fan). What might this be caused by?