I think all of us, if we are honest, are perhaps a bit less 'on the ball' in terms of fast recall of exactly what every process is and exactly what needs to be done next and how on the first day back after a holiday.
However, we are also rested and have lots more energy, enthusiasm, bounce and new ideas.
I don't think we'd like to hear ourselves described as 'regressing' when we are like this. 'Taking a little while to get back into work mode', perhaps?
I think that the same is true of children after the long holiday. For a few days they are a bit less sharp and a bit less quick, especially in tasks that require instant recall of facts. On the other hand, they have lots of energy to try new things and spark new ideas.
So a 'snapshot' test done on day 1, requiring instant recall, might seem to indicate that children weren't quite where they were at the end of the summer term (in the same way as an assessment of someone at work on the first day back after a long holiday might indicate that they weren't quite as quick at remembering particular telephone numbers or particular processes) but that continuous assessment over the first few weeks would give a different picture particularly if energy / ideas / enthusiasm / engagement were being measured.