Not every kasrah immediately before a still ra is original. At the beginning of a word with hamzat al-wasl, an initial kasrah may arise only to make pronunciation possible. This temporary kasrah does not produce the usual thin ra; the ra is heavy.
«اِرْجِعُوا» — when beginning here, the vowel of hamzat al-wasl is temporary and the still ra is heavy.
A purely visual rule of “kasrah always makes ra thin” would therefore misclassify this example. The nature of the preceding vowel is essential, not just its sound at that instant.