Child bone relative location shifting

Development Character & Animation
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1

I have two virtual bones on character mesh (also tried with a regular ones, same result), VB Sight and VB Target, that parented to VB Sight.
Initially, there is no offset between them. When i add some offset to VB target bone, and start rotating VB Sight bone, VB Target bone relative location (offset between this bones) slightly changes.

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Why this can happens, since i assume this value should stay constant, since VB Target is
a child of VB Sight, and also target offset stays constant and reapplies to VB Target bone every tick?

2

UPD: Partitially i solve the issue by converting this offset into world space, and apply to bone in world space.

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Hovewer, this cause a one frame lag now, and i try to change skeletal mesh tick group to PostUpdateWork, but without any effect.