I do not see it clearly,
Well, when we put an envelope in that input the range varies from 0 to 1 (float), but I don't see in line 90 the transform from the range 0 to 1 to an integer int16_t.
void AudioFilterStateVariable::update_variable(const int16_t *in, const int16_t *ctl, int16_t *lp, int16_t *bp, int16_t *hp)