The Bug:

The issue was introduced in 23w31a due to resource pack changes occurring in this version. In versions below 23w31a, both the icons within the accessibility and language buttons could be textured differently depending on whether the said button is highlighted or not, however, this is no longer the case.

Since the separation of the icons and buttons, the icons can't be given a variant for hovering. If you look at other buttons like "locked_button", their icons are directly placed on the button texture and are not separate into /icon file. This makes hover icon impossible to make it different when hovering the button.