If you don't want to remove the appmenu-qt5 app, another workaround to make the shortcuts work again is to launch texmaker without global menu (that is with the menu bar inside texmaker window).
You can do this by simply changing the way texmaker is loaded (using alacarte for example) from
texmaker %F
to
env UBUNTU_MENUPROXY= texmaker %F
If you don't want to remove the appmenu-qt5 app, another workaround to make the shortcuts work again is to launch texmaker without global menu (that is with the menu bar inside texmaker window).
You can do this by simply changing the way texmaker is loaded (using alacarte for example) from
texmaker %F
to
env UBUNTU_MENUPROXY= texmaker %F
That works perfectly on my computer at least.