I have Series 6 and 3. Bothe running watchOS 7.1
With Sleep mode enabled - Series 3 measures and reports the RHR right after getting up but Series 6 does NOT.
When I spoke to Apple over phone - they said - RHR isn't supposed to measure/reprot HR measured during sleep.
In fact if you go to RHR section in Health - the "About RHR" section explicitly says - "...RHR does NOT include your HR while you're asleep..." (see attached image)
So if this definition is to be trusted then Series 3 has a bug while Series 6 is doing what the Apple documentation says - i.e. not to include HR during sleep.
I suspect putting Sleep mode on - explicitly tells the Watch to "Stop including HR info now that iAm sleeping". But somehow this instruction isn't obeyed by Series 3 - maybe a bug.
But it's quite confusing.