When a panel is placed at the bottom of the screen and set to auto hide, it will not auto hide when there is a second monitor placed below. Screenshot of my monitor layout attached below. Please note the primary display and the layout of the two monitors. Should be fairly easy to reproduce. Running the latest stable release. Thanks. https://i.imgur.com/1QVN5xx.png
This is intentional. Auto-hide relies upon your mouse touching the bottom pixel of the screen to show the panel. This is trivial when that pixel is at an edge with no other screen underneath or beside it. But when there is another screen there, that key pixel becomes almost impossible to hit. If we allowed this, we would get bugs from people saying, "it's impossible to show my hidden panel when placed on the bottom of a screen with another screen below it" If in this situation we increased the size of the zone that will show the panel, we would get bugs from people saying, "my hidden panel that's placed on the bottom of a screen with another screen below it displays too early." The only sane approach is to disallow auto-hiding a panel that is on a screen edge next to which there is another screen. My suggestion is to move your panel to a different edge.
Well that's unfortunate. Wouldn't it be possible to have auto hiding working if there's a gap between the two screens?