Spikes that have no relation to actual heart rate are, as you implied, usually down to contact pressure on the skin. It can be too heavy as well as too loose. It gets worse with darker skin, older skin and definitely tattoos.
You can try wearing it inside the wrist or on the other wrist if it occurs regularly.
This is typical of a spike, characteristic plateau on the graph (Strava graph)

When I had a Polar Watch and it happened much more often I took to wearimg my wife's Polar Watch on the pother wrist to make sure it was not me! (Quicker than stopping and taking carotid pulse) Once I knew it was skin malfunction I was not so worried.
I think Apple Watch for me is the most reliable but no optical system can work perfectly on all skins in all lights and weather conditions.