Right - I think this is the case but I think there is also a second rule which is hardcoded at a lower level, where if a fare can price in a given cabin on any sector of the fare component, it may delegate to the unrestricted inventory class of any lower cabin on any other sectors of the same fare component. The biggest example of a loophole for this I saw was a couple of years ago when many Express flights used a short-scale F/A inventory set, so for example the T/UPDI fare booked into A. I could then confirm it into C (interpreted as Business full-fare subordinate to First, even though that was wrong) on a sector which had neither T nor A space (was like F5 A0 C5 D0 .. Y9 T0). This worked even on e.g. MSP-ORD-SFO, where MSP-ORD was the "prime" segment with the A space.

My theory is that the T/P fare here will behave as "T REQUIRED WHEN AVAILABLE ... Y REQUIRED" for a subordinate segment where it can confirm in P on a prime segment. But I agree that for the current /UPDI fares, the rules require prime only but the fares clearly do not price as such. It's a bit of a mystery still.