Isolated durations on rests

[Noeck]

Hi,

is this behaviour intentional?

  \version "2.20.0"
  { r4 8 a b }

The "8" is equivalent to c'8. I think I understand what happens: the
isolated duration is given a default pitch c'.
What I would expect is that it is another rest, equivalent to

  { r4 r8 a b }

Was this just easier to implement that way for isolated durations or is
there another reasoning behind that behaviour?

Cheers,
Joram