When an F-Curve doesn't contain any keyframes anymore but it still exists, the F-Curve's value is not recalculated and flushed anymore if the F-Curve's value will not change. That is, if the F-Curve doesn't have any other data, i.e. drivers or generator-modifiers, which would still change its value, it wouldn't be recalculated to have a value of zero.
This solves the problem of deleting all scale keyframes, whereby objects/bones would appear to 'vanish'
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