A vowelled ra bears fathah, kasrah, or dammah. Its first rule is direct: when ra itself has fathah or dammah, pronounce it heavily. A previous letter’s kasrah does not override this direct ruling.
«رَبِّكَ» — ra has fathah: heavy.
«رُزِقُوا» — ra has dammah: heavy.
Heaviness must still preserve a single ra. Excessively trilling the tongue would misuse ra’s inherent capacity for takrir. The prescribed change concerns resonance, not the creation of additional consonants.