Why in this case there isn't an agreement? Should it be
il nous a laissés partir.
When laissé is followed by an infinitive, it's not wrong to make the agreement, but the latest spelling reforms recommend against it. The logic is that the direct object is actually the verb "partir", not the word "nous".
Nous can be an indirect or direct object pronoun.
As I understand it, the french means "He has let us go" so yes, nous should be the direct object and because it appears before the verb; should require concordance with the past participle laissé.
I agree with you and suggest you file a possible error report.
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