After a bit more digging, I believe this only affects the simulator. It seems to occur anytime an overlay on a map is rendered. When the map position changes for any reason, e.g. scrolling the map, the leak gets bigger. Obviously this becomes a more significant issue if the map position changes automatically as in Henrivb's case and mine. You can see the leak grow just by rendering one overlay on a map, then moving the map around. I've filed feedback to Apple on this as well.

I see this memory leak as well in iOS14.4. As stated above, it doesn't appear to leak in 13.7. Has anyone heard more information regarding a fix or workaround?