Good jump on the research.
I try to do what I can, before asking other people to spend their time on me :-)
I don't have a direct answer, but check the version(s) of LibreOffice, Dropbox, and possibly some of the other packages you've already mentioed. Perhaps your issue is something that's been fixed by a newer package in Ubuntu and not yet fixed in the corresponding package in Arch (or something that's been broken by a newer package in Arch and yet merged into Ubuntu). Maybe a release note or a patch will jump out at you.
That is a solid idea! I'll see if maybe I can find a version mismatch and downgrade accordingly. I already checked Libreoffice's bugtracker for this (which would indicate there's a patch incoming), but haven't found any entries, so I'll file one. Thank you. -- "That gum you like is going to come back in style."