Panel does not disappear automatically

I have a system with XFCE 4.20, two panels defined:

• One panel is set to never disappear.
• The other panel, when set to disappear 'intelligently' or 'Always' (same result).

The second panel does NOT always disappear.

Also, there is a very fine red line around the panel, as if the panel is still in the 'configuration' state.

What can I do to help debuging this?

I tried to compile the latest versions of the plugin, but it doesn't seem compatible with Slackware's slackbuild-64's system (I suspect the slackbuilds are not adapted for Meson?).

Tried to compile it 'manually' too, but my knowledge of the new build system seems lacking.