Device registry does not clean itself when no entity linked to it

The problem

HA does not appear to clean the device registry when no entity is linked to it.

Home Assistant Core release with the issue: 0.115.X, 0.116.X, 0.117b3
Last working Home Assistant Core release (if known):
Operating environment (OS/Container/Supervised/Core): Container
Integration causing this issue: plex, but I think it's core
Link to integration documentation on our website: https://www.home-assistant.io/integrations/homeassistant/

none

none

Entry manually removed