OK, I found the problem. The problem is with the libreoffice-kde5 integration package. Seems like this package has always been problematic, and in this case it is preventing the ALT key from working. Removing this package does restore ALT key functionality, but introduces another problem: LO comes up in a window that is some sort of GTK thing, and the menu bar, dropdown, etc fonts are too small to see comfortably. I have not been able to figure out how to change them. The KDE menus for changing GTK application font sizes do nothing, so LO is basically unusable in this configuration. Not sure how to proceed here, but the original problem does seem to be solved, even if not usefully.
Edit: Thanks to a user on another forum, I got the font problem solved. Apparently Libreoffice switches to a native interface, which is not really gtk, when the integration package is removed. Installing the libreoffice-gtk2 package fixed the whole mess.