Ok, thank you, that fixed it. I really had specific values in the buttons:
Removing these made the buttons sync to the group button's settings.
However, I still wonder what the buttons looked so weird in the first place, with the settings above they should have nicely rounded corners, shouldn't they? (They even retained the nicely round corners after I put 12% in the boxes again, effectively setting it back to what it was before. Why do I need to wiggle with the numbers to get the system to work?)