Greetings, it seems messa di voce hairpins are not following engraving rules.
I could be wrong, but this may be related to the other unique behavior of messa di voce (playback, dragging). Try adding the cresc and decresc separately, and it will probably work. (Not that it should be necessary).
Messa di voce hairpins are not the same as hairpins, so they don’t follow all of the engraving options for hairpins. I’ll make a note of your desire that they should follow this one.
Personally I would not change this behavior. Restarting that hairpin over the system break would be confusing, as it would really look like two separate crescendo hairpins.
I suppose you’re right. That said, it looks even more awkward when it’s the decresc. rather than a cresc.