Maybe I have not expressed myself clearly enough:
When using the MicroCode core, the delay(1) is needed.
When using the attiny13 core it is not needed.
It seems that the adc needs some extra time/action in order to function after a wdt controlled sleep when using the MicroCore.
If a delay(1000) is used in place of the wdt 1 second sleep it also works, probably because the adc is not shutdown in between.