Middle-click on the back button triggers the titlebar action
Epiphany version 3.34.0-13-g13c2a83d with WebKitGTK 2.26.0 on FreeBSD 12.0
In gnome-tweaks, if "Window Titlebars → Titlebar Actions → Middle-Click" is set to an action other than "None", middle-clicking on the back or forward button in the header bar of Epiphany triggers both the button action and the titlebar action. For example, if you set titlebar action to "Lower", middle-clicking on the back button not only opens the previous page in a new tab but also lowers the browser window. It makes it hard to use buttons on the header bar when a titlebar action is set because the browser window is hidden under other applications as soon as you middle-click the button.
I am not sure if it is a bug or it is expected to work in this way. Firefox doesn't put buttons in the titlebar or the header bar, so it isn't affected.