On Mac, launcher process uses 100% CPU after game is launched

Launcher "Version 598 (1)" on Mac starts using 100% CPU when the game is launched.

Some process called "Minecraft Launcher" stays running. Sometimes from Activity Monitor, I can quit that process, but sometimes I have to force kill it to make it go away.

This is the case whether or not the "keep launcher open" option is selected. If it is, quitting the launcher doesn't make the CPU hog process go away, so this is apparently some other process spawned by the launcher that is not the same as the launcher app.