Is this oven thermometer malfunctioning, or is my oven misbehaving?

I just bought an oven thermometer, and it’s not behaving consistently… eg, I set the oven to 350, and at first the oven thermometer was reading just over 500 degrees; a little bit later however, the oven thermometer was reading around 400. Is it common for ovens to change temperature throughout the course of running? Or, are there any pro tips on using an oven thermometer? Thanks!