I have three themes configured under the themes: component in my configuration.yaml.
If I go to Services > frontend.set_theme, I can set any of the three themes and the specified theme loads when the service is called.
If I go to Configuration > General > Set a Theme, I can also choose any of my three themes as expected.
However, after HA has been running for some amount of time (could be anywhere from 30 minutes to four hours), the “Set a Theme” dropdown is no longer present in Configuration > General.
I have checked the system logs, and nothing related to themes or frontend is shown. I do not have any automations or scripts that work with the themes - they can only be changed through the dropdown or the frontend.set_theme service.
Any thoughts?