There could be a few reasons responsible for those foam insulation cracks but external elements such as heat and cold often related to improper cathedral ceiling ventilation
should not affect foam insulation integrity. Assuming that it was properly installed, as soon as it dries completely, it should not expand & contract with temperature changes.
The cracks you are describing are quite large, and I’d rather attribute them to improper foam installation / insufficient amount of foam between the structural members than the ventilation issues. Structural and design problems of the roof frame may be responsible as well.
Another thing that I recall from my inspections which observation usually applied to foreclosed unfinished properties was hiding of some defects (often structural) in order to sell the property.
In your case; if you can see a 1”-3” wide crack in 8” of foam insulation and there are no signs of cracking, separations, loose finishing tongue and groove ceiling boards, it’s a shrinking foam case (extreme) or maybe the ceiling boards have been installed to conceal some problems.
I can’t visualize the floor plan of this house but maybe I get it right; standing on the outside of the structure look at the surface of the parallel walls supporting section of the roof you’re asking about. Look along the roof edge, gutters (if there are any) directly above those walls and see if there’s any noticeable bulging. You can also measure room’s interior dimensions along the same walls / at the roof frame / wall level, just to see if everything is square as it should be.
I don’t see how you can convert this roof into an unvented system using cellulose insulation. For such roofing system to function properly instead of being consumed by mold is to apply closed cell spray foam directly to the underside of roof decking surface. Filling that empty ventilation space with cellulose insulation will trap moisture and result in wood deterioration.
Spraying the insulation into the remaining space between the roof decking and the existing, cracked foam layer does no seam possible either but you’d have talk to a contractor about it. Maybe they have some equipment capable of delivering foam insulation into such spaces without causing roof decking deformation.
Please, let me know if you’ll learn something from further evaluation of this property.