Clearly a bug, see https://gitlab.gnome.org/GNOME/gimp/issues/1466
Workaround: if you do any further change to the selection (move or change the size by dragging a border) the new radius is taken in account).
PS: created a new issue: https://gitlab.gnome.org/GNOME/gimp/issues/2665
Workaround: if you do any further change to the selection (move or change the size by dragging a border) the new radius is taken in account).
PS: created a new issue: https://gitlab.gnome.org/GNOME/gimp/issues/2665