Pently misbehaves with zeros in volume envelopes

[NovaSquirrel]

durations stick
notenames english

instrument Triangle_Short
  volume 15 0 0 15

song World_5
  time 4/4
  scale 16
  tempo 225.00

  pattern pat_1_2_9 with Triangle_Short on triangle
    absolute
    g,16 b,16 d16 g16 b,16 d16 g16 b16 d16 g16 b16 d'16 g'16 b'16 d''16 g''16 b'16 d''16 g''16 b''16 d''16 g''16 b''16 d'''16 g'''16 d'''16 b''16 g''16 d'''16 b''16 g''16 d''16 r1 w1 
  at 1
  play pat_1_2_9
  at 5
  dal segno

This score is supposed to produce a warbly effect on the triangle, but instead is barely audible. No matter how much stuff you have past the zero, it seems to get ignored.