The Factory/Harmony Tonal preset does not contain any support for choosing instrument groups. It contains no allow_group or disallow_group calls.
When you attempt to use the preset with the Factory Library Orchestral/Strings presets, such as Cello Solo or Viola Solo, the group selected is always forced to Tremolo. How about some code to select Sustain? disallow_groups($ALL_GROUPS) allow_group(0) (for Sustain) does not work. If you Bypass the preset, all groups work fine, so the preset is at fault.