I encountered this problem today : Some voices (always the same) seem to have a different 'scaling' on the filter envelope. So the filter will open slightly less on those. At a certain point, if you set the LPF relatively low, it feels like they disappear. It makes for a pretty un-even sound unfortunately, and is hard to work with. It doesn't happen if you use the SVF in lowpass mode though, so it seems it's a LPF issue. Considering downgrading to 1.3 to see if this helps (also considering my other post regarding volume difference in 8 of 16 voices).
Again, a video to show the issue:
https://www.dropbox.com/s/omphtvsv64mn3 ... 6.mov?dl=0