Hard to say Eragorn, without more information and pictures.
35 years ago in Northern Maine, I was building duplexes and quadplexs, and the roofs were all stick built with 2x6 rafters or homemade trusses two feet apart with 5/8" CDX plywood on the roof for sheathing.
Was never an issue with spongyness.
Maybe they had used code minimum on the one your talking about, which is 3/8" :mrgreen:
Well, blowing up that first pic to 400%, the plywood through the hole sure don’t look like 5/8"
7/16" at best.
Ironically, I inspected a home built in 1975 today, and it happened to be a modular home and found the inspection sticker underneath the sink cabinets. Same as your picture and spongy when I walked the roof.
Plywood was very expensive in 1973-77. A lot of contractors cut down to the thinner plywood and that is what you got.
I could not find a grade stamp today either. But it definitly was not 5/8" plywood.
The use of h-clips had not been out yet either.
Should be fine. But would strongly recommend they not have double layer of shingles.