You can also restart the keyboard with systemctl --user restart maliit-server.service
I've never encountered this issue with Waydroid before. Just a wild guess, can you please check in Settings >> Language & Text and see if there's perhaps an external keyboard detected. I guess you can also see this if the keyboard icon appears in the top bar. I'm thinking maybe Waydroid introduces an external keyboard, for some reason, and onscreen keyboard is disabled when this is the case