/alternative slur bug

[bb]

\version "2.20.0"
\relative c'' {\repeat volta 2 { f1 ( |   }
  \alternative { {f2) r2}
                        { f1) }  } }

No slur with the second alternative, warning that slur cannot be ended

Warnung: slur kann nicht beendet werden
    { f1
        ) ( |  f2 ) (f4) r4 }