This is a problem with most Hybrids and EVs, as the brakes don't get used a lot, so dirt will settle and bare metal will rust, especially the rear ones; Some cars are programmed to occasionally skip regen, but mostly it's up to you to occasionally brake harder than regen, or, here possible, sensibly use the handbreak or e-break where possible; Had this issue with my Toyota Auris Hybris as well, so happens to the best of them; It's also the reason more EVS and Hybrids nowadays go back to rear drums, as those are less prone to these issues.