you will need to select the random option and use the next brush key it will automatically pick the random brush from the brush set.
and if u press the next key again it will again pick the random brush from the current brush set.
You can check the short gif tutorial here!
Also the GUI issue i will be checking as soon as possible! I will try to recreate in my system and see how to resolve it !
Thanks if you have any issues let me know!