iOS 14 developer beta

This is typical of all new IOS beta builds. It's normally because there is some indexing and debugging turned on that normally wouldn't be running. You can let it go for a while and see if it resolves, but your best bet is downgrade and wait till about the middle of July. The first few builds are so unstable that using them on your personal phone isn't recommended. If you need testing done use the simulator. The overheating normally goes away around that time after they start cleaning up the code.

lennyj22 has some good points, but before you change anything, would you please file a bug report using the Feedback Assistant app on that device or using the Feedback Assistant web page. That will send info to teams that can analyze the issue(s), maybe provide a workaround, and hopefully make it better in a future release.