I wonder if this is case dependent - and it varies depending on content of the other staves. It seems like Dorico gets into a status quo of spacing in Galley View and, Galley View by design doesn’t really apply casting off. (Obviously making some educated assumptions here). 
Could this be OS dependent?
I have macOS High Sierra.
And today I had problem with the same diminuendo even in Page View.
And also in another case I had problem with proper placement of the dynamic. Please see attachement.
How can I move dynamic to the end of first bar? But I don’t want to move by rythmic grid by clicking „alt+cmd+arrow”, because If the spacing will change, the position of dynamic could change also. I don’t want to have „mp” linked to the specific duration in the bar (e.g. last 8th or last 16th or last 32th), but I want to chave „mp” always at the end of duration of the tied notes in the first bar, no matter how the global spacing will change.
I hope I am clear what I want.
Edit. Maybe this is not good example. I can do it by “shift+alt+arrow”, but it’s not working a couple of bars later (see second attachement).
dynamic 3.zip (1.45 MB)
dynamic2.zip (1.16 MB)
For Galley view in the OP’s original question, if you adjust the Engraving options>Dynamics>Gap between hairpin and barline to be 1/8 and you will see the diminuendo hairpin.
There is a bug whereby under some specific combinations of gradual and immediate dynamics pressed together within a short rhythmic duration, sometimes you will find that the dynamics are either displaced or, in the case of gradual dynamics, if they end up only allocated a very small amount of space, they will eventually refuse to draw at all (so as not to risk them looking like an accent). This is something we have on our backlog to fix in future. You should find, in the meantime, that giving things a bit more space (e.g. using the note spacing tools in Engrave mode) will improve matters.
Thank You, Daniel, for Your advice in this case.