Can confirm, it looks like the bookmark will be moved to the next line of the selection, like
having the following text (O indicates the bookmark, numbers obviously the line margin)
O 1 hello hello 2 world world 3 hello hello 4 world worldbookmark on first line and selection is on complete first line
(eol included so that cursor is on first position on second line)
then by pressing alt+u it will result in this
if selection is line 1 including line 3 (cursor is on first position on line 4) results in
1 Hello Hello 2 World World 3 Hello Hello O 4 world worldand by selecting all text, bookmark will be deleted
1 Hello Hello 2 World World 3 Hello Hello 4 World WorldCheers
Claudia