I have downloaded Roboto Flex via the font-manager, despite it is working, there is an error (probably the reason I can’t use the slant settings), which might be caused by its name including a blank inbetween:
Thank you for bringing this to my attention, sorry for the lack of reactivity!
The reason the slant setting isn’t applying is because the value you have set (58) exceeds the accepted values for slant which should be between -10 and 0.
Be sure to experiment with Roboto Flex’ properties here, and see the appropriate values to set.
Once you have corrected the value, kindly let me know if the issue persists.