Cross-spectrum connection is not supported

[Mr.Z]

the line is missing：

this is xml:
test_line.xml.zip

(*
  gmn code converted using libmusicxml v.3.21
  and the embedded xml2guido converter v.3.2
*)
{[ \staff<1> \set<autoHideTiedAccidentals="on"> \auto<autoInstrPos="on"> \instr<"钢琴"> \accol<id=0, range="1-2"> \barFormat<style= "system", range="1-2"> 
   (* meas. 1 *)  \clef<"g2"> \key<0> \meter<"4/4", autoBarlines="off", autoMeasuresNum="system"> \stemsUp \beamsOff a1/4 \stemsDown \beamsOff d2/4 \stemsDown \beamsOff b1/4 \stemsUp \beamsOff e1/4 ]
 , 
[ \staff<2> \set<autoHideTiedAccidentals="on"> \barFormat<style= "system", range="2"> 
   (* meas. 1 *)  \staffFormat<distance=13> \clef<"f4"> \key<0> \meter<"4/4", autoBarlines="off", autoMeasuresNum="system"> \intens<"f", dy=19hs, dx=0> \stemsDown \beamsOff \fingering<dy=-7hs, dx=0.2, text="4">( g0/4)
 \stemsDown \beamsOff a0/4 \stemsDown \beamsOff b0/4 \stemsDown \beamsOff c1/4 ]
  }

[Arshia Cont]

Thank you @zhwei8090 . This is actually a missing feature in GuidoLib where Guido is unable to render Slurs between different voices. So I decided to eliminate those Slurs when converting xml2guido (otherwise, orphaned Slurs lead to confused rendering in my experience).