Trying to understand why this is happening with the staff spacing of the last system, and how to fix it?
That last frame is below the threshold in Layout Options > Vertical Spacing > Vertical Justification, so Dorico is not adjusting the spacing of the staves at all.
I suspect that what you want is not to have those staves spread evenly over the whole page, but only over half the page (matching the first system on the previous page). You could accomplish this by spacing the staves out manually, or by dragging the bottom margin of the frame to the middle of the page and letting Dorico handle the spacing.
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Brilliant, thank you !
I found @benwiggyâs vertical spacing guide helpful for understanding stuff like this too:
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Ah, this is very helpful! Although, it shows variables for 2 systems - my issue was an orphan system on the last page - changing the frame size seems like a more âDoricoâ solution than manually moving the staves.
The real Dorico move would be to create a new page template that looks exactly like Default but with a shorter Music frame. This way, you avoid the override created by changing the frame of the Default page manually 
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The REAL Dorico move would be to set your âIdeal Gapsâ to larger values, so that the finale systemâs stave are spaced out more like the previous pages.
Remember that the page is spaced according to the Ideal Gaps and then it is stretched (justified) to fill the page (if it meets the percentage criteria).
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This is exactly what I told them over there on Facebook 
Well, you donât always have enough systems/notes/pages for this to be THE solution. But itâs obviously the FIRST solution to aim for
Mine is for those scores that wonât let you end with a full page (and they do exist!)
In any case, neither of our solutions involve letting an override on the file ^^
A cheeky Note Spacing Change usually fixes that! 
Usually yes. But not always!
âŚso as to continue flow into another (or however many) system(s) to fill up the last pageâŚ?
Do you have a default setting for this? I understand itâs gonna be different depending on how much is going on above the staff etcâŚ
I found that changing it didnât do anything, but setting the minimum gap did the trick. It had been at 1 space (?!) Setting it to 6 did the trick.
Thatâs likely to have âunforeseen consequencesâ; I wouldnât recommend it.
Generally, you want the Ideal Gaps to be âthe lowest distances you will needâ. Because the page gets stretched to fill the page after those values have been applied. So on a page that doesnât get stretched, those are the values that will be used.
Setting Minimum Gaps to large values will just put massive margins of white space between each staff.
Iâve never understood why not the âminimumâ distance but the âidealâ distance would be âthe lowest you will needâ - which is literally the definition of a âminimumâ.