Why does kasrah not automatically lighten rāʾ in رَبِّ ارْجِعُونِ?

Tafkhīm and Tarqīq — Alif, Divine-Name Lām and Rāʾ

• The kasrah is in another word
• Rāʾ has ḍammah
• Long yāʾ follows
• It is a stop