The way I see this is that the exposure time isn't a binary thing - if you're at say 25 days exposure you've done no damage but at 32 days you've got garbage. It seems to me that any UV exposure will take life out of the pipe at the far end. 30 days is a short window and tells me that any UV exposure is bad.
I agree that all PEX should be UV protected both at the manufacturing facility, the distributor, the local store, and during and after installation. Even the big box stores have bright LED lighting which puts out a surprising amount of UV light. Probably the safest way to buy PEX is in straight lengths as long as it's boxed, particularly type A.
I wonder how much PEX was stored outside at the plumber contractors or in the back of a pickup truck, and wonder if that might explain the relatively small amount of PEX that fails overall but would also account for the many leaks that seem to occur all at the same location!
John