As users, all we can do is speculate. This particular function hasn’t previously been discussed at length, here or on the Facebook group. The function’s only existed since Dorico 3.0 so those of us that are old hands likely don’t depend on it.
But to it not being a bug, notice how the System fullness indicator does update when you adjust note spacing, and the entry in the manual (here) tells you that that indicator updates automatically when you adjust note spacing. Then notice that the entry for the Frame Fullness Indicator (here) doesn’t say anything about this indicator automatically updating. That’s the clue that this is working by design, and thus isn’t a bug. It might be something you don’t like, but if it’s working as the developers designed it to work, it’s not a bug.
As to why manual adjustments don’t bring forward following systems, can you imagine what a nuisance it would be if you dragged the staff spacing closer on page 1, watched Dorico move a system from page 2 to page 1, then you thought “well I’ve got a little more room at the top of page 1 so I can drag those a little bit bigger”, then watched Dorico shunt the bottom system back onto page 2. Worse still would be if you were editing the staff spacing on the top system of page 2 and Dorico decided it could shunt that system onto page 1. No thanks!
edit: I see that Sibelius does exactly this. After some three years of barely touching Sibelius I’d forgotten this behaviour. Bearing in mind that the Dorico developers previously worked on Sibelius (in some cases for upwards of 12 years), I suspect they realised that they could improve on Sibelius’s staff spacing behaviour. If you’re a current or former Sibelius user, please don’t expect Dorico “to do things in a particular way because that’s how Sibelius does it” - a great deal of thought has gone into the development of Dorico (which is not to say that a great deal of thought didn’t go into developing Sibelius; it’s just that Sibelius carries decades of legacy behaviour and the Dorico team had the chance to rethink stuff)…