slur colliding with key signature

[Nicolas Sceaux]

the slur goes through the key signature:

\version "2.3.24"
\layout { raggedright = ##t }

{
    e''4 b'(  
    \key bes \major
    ces''2)
}

In the original edition, where 3rd-line notes seem to be stemed
up, the slur goes behind the key signature change.

nicolas