With the new autohide animation code, panels with the background style set to solid color do not intelligently or always autohide. It works ok with the background style set to None. Maximized windows also display over the panel in this scenario.
Thanks for noticing - I would have never guessed that there is a side-effect coming from the background drawing...
Ok, so the root cause is we're not setting any borders when using custom backgrounds (color or image) so the autohide animation code is unsure about the panel's location. I'll try to fix that after next week...
Simon Steinbeiss referenced this bugreport in commit b39978985b90c5c3478aea8fab484d2f2707c4ac Fix autohide with bg color or image (Bug #16064) https://git.xfce.org/xfce/xfce4-panel/commit?id=b39978985b90c5c3478aea8fab484d2f2707c4ac
It turned out to be easier than expected, so it's fixed now.
Thanks Simon. Patch fixes the issue.