Autothrottle retards on derated take-off in 747

If I set derated thrust for take-off in my 747-400, the autothrottle pulls the
throttles back just as I'm reaching the middle of my take-off roll, typically
right around V1. I have to pull the A/T and advance the throttles myself or
risk a crash.

Can anyone explain what I'm doing wrong? I have no trouble with normal
take-off thrust. I have RTO set; is the aircraft somehow deciding that I need
to abort the take-off, or what? Usually I'm right about at rotation speed (V1
and Vr being very close with light loads), and I can clear the runway but I
have to press TOGA or advance the throttles myself to counteract what the A/T
is doing.

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