I'm way behind the times, but I assumed that if someone came back full-time part way through July, the grant would be worked out in the same way as under CJRS v1. However, it looks like it's calculated in the same way as someone who was furloughed for the whole of July, coming back part-time part way through July. So if someone who'd been furloughed since say 1 April came back to work full-time on 2 July, you still need to calculate the usual hours and the hours worked for July.
That's what HMRC's calculator suggests. It's also backed up by the definition of "Flexibly-furloughed employees" in the Treasury Direction of 25 June (particularly 10.2(c)(ii).
What a pain! Am I missing something (other than a fully-functioning brain)?