Can't modify multiple shapes on a chord

[Paolo Prete]

Hello.

Please consider this (tested on 2.19.84):

\new Voice <<
  {  \shape #'((0 . 0) (0 . 0) (0 . 0) (3 . 0)) Tie c' ~ c' }
  {  \shape #'((0 . 0) (0 . 0) (0 . 0) (0 . 0)) Tie c'' ~ c'' }
>>

If I modify the shape on the first tie, changes are applied on the second
tie. Is this a bug? If so, is there a fix/workaround for this?
I tried  \partcombine too, and the issue persists.

Thanks.

Best,

P