Well since it seems to be filling in details and involves Beatrice, who to this point has been mostly absent, I can't see anyone thinking of this as filler. Especially if they don't know what is going on (how can one recognize "filler" if one does not know the plot?).
And since this is a "before everything happened" story, we don't have any sort of "wait their alive?" moments like Higurashi had following the first arc.
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