I am trying to book a multi-stop ticket:A-B-C, D-E and F-G-H

I like to book with GPU (request upgrade on the time of booking). If I choose the above routing, it shows that PZ is not available in A-B-C and not F-G-H.

However, if I book A-B, D-E and G-H, with GPU, it shows PZ availability in both A-B and G-H. Of course I can book this and later book B-C and F-G, but it will ended with two PNRs.

My question- If I choose to book the first option (original routing), and I later apply the GPU, will the availability of PZ remains the same? Also, why it does not show availability if PZ is available for the first segment?