I prefer wingpanel not being transparent, and have set org.pantheon.desktop.wingpanel.use-transparency to false. Several times recently, and just after running updates today, my wingpanel became transparent.
Using dconf Editor to check the setting, it still showed false (not transparent). Once I checked the box and then unchecked it, wingpanel was again opaque.
Is anyone else having this issue? Is it update related or was that a conincidence? Is there a different setting I should be using? Thanks.