i fixed this currently via custom.css with :
.t4-off-canvas-body .deeper::before {
position:absolute;
top:0.35rem;
right:0.3rem;
content: '\f35a'; /* arrow right */
font-family: 'Font Awesome 5 Free'!important;
font-size: 1.5rem;
color:#551285;
}
...But the "current" or "active" class for that point is not be set, so you can add one icon , but can´t change it to a arrow down, or anything like that, because of that bug that the classes there are not set in 1st-Level...
..and first point stays always on css class "current" - probably the same bug..