Strange Offset for an animated Alembic in the Proxy Node with Playback to Once

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Hi,

we have an animated Alembic file with 87 resolved frames. We want the animation to start at frame 1000. With the Playback type set to Once, the offset needs to be -1000 to get the animation starting at frame 1000. Why do we need to set a negative number?

TIA
Lars