Yours is a very good question
In many grammar books that 'rule' appears, but then, when paying close attention to texts in general, you will find many sentences like the one you've posted.
I would say that the use of a comma in sentences like this is a 'preferred form' which may mark a sense/breath group (a rule prescribed to avoid misreading of sentences that are too long, which is not the case of this sentence, or perhaps to avoid misunderstanding an ambiguous sentence)
We have also found cases like this at university (with a subordinated clause at the beginning of a sentence and without a comma at the end of the clause), and also cases of non-defining relative clauses without commas.
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