Yes, but more so with PUR boards. That's why they ought to be slightly undercut, and squirty foam all around edges. It's even better looking under "well insulated" suspended floors, to find a nice layer of kingspan covering the solum, having dropped out from between the joists. Meets all the regs on paper, but a few years down the line, it's back to living in a timber shed.

Yes I have commented on this a few months ago but cannot remember what thread it was under. I had some delivered warped and with concave edges just over 12 months ago they got sent back.They were due for the floor. Seems they had been exposed to the sun in the builders merchant yard. I was made aware of this problem a few years back when a builder friend went to extend a roof he had build some 10 yrs earlier. He found the same result as you
When I built my roof I put 170 mm or rock wool between rafters and then 100mm over clad the rafters with quinn therm taping the joints. That way the foam insulation was protected from most of the extreme heat.
There is a company that make special tape that fills the gap between the rafter and insulation but have no idea if it would cope with the shrinkage you describe.

Would multi layers with staggered joints help to reduce the problem caused by shrinkage gaps?

So the next question would be is the shrinkage a one time thing that stops after a certain point? If so should only age cured be installed

Posted By: fostertomAny suggestion this applies to EPS too?

I wrote: Not as far as I know. Providing you start with 'aged' sheets, not ones newly made. I think the conventional wisdom is they need three months of aging. No evidence of it in the stack of EPS in my garden.

And then I read: https://www.buildingscience.com/sites/default/files/migrate/pdf/PA_Foam_Shrinks_FHB.pdf

So now I'm as confused as you. Maybe he started with newly-minted sheets?