مَاتَ فُلَانٌ، رَحِمَهُ اللَّهُ
So-and-so died; may Allah have mercy on him.
The first clause, مَاتَ فُلَانٌ, reports someone’s death. The following رَحِمَهُ اللَّهُ may function as a prayer for divine mercy. Their differing communicative purposes can justify leaving the clauses separate.
The past-tense morphology of raḥimahu Allāh should not automatically be equated with an ordinary assertion. In the context of supplication it conveys a wish or prayer. The distinction between grammatical form and rhetorical meaning is therefore decisive.