I tried to circumvent the ctrl modofier in such a way, that I look into the event's previous value and delete/save as necessary. Now, there is also always one option selected
I modified the callback as follows:
if strcmp(fig.SelectionType,'normal') % to preserve ctrl behavior
please note that with the current state, the bahavior with ctrl left click changes and does not work as intended anymore.
EDIT: edited code to properly preserve ctrl click behavior