The tz offset field is prefilled with '+0000' in the change values + attributes page regardless of the TZ the sensor was setup with. This makes switching between local times and '+0000'/utc times necessary when testing/manually updating the sensor.
When the TZ offset field is emptied and left blank, the timezone defaults to '+0000'/utc regardless of what the sensor was setup with or what the last value used was.
Expected Result
The timezone used to setup the datetime sensor should persist and be the "default" time zone for that entity
When the TZ offset field is empty it defaults to the HA Timezone instead of '+0000'/UTC
Issue
Sort of two issues in one here I think:
Expected Result