It seems, historically, only polygons from an ENTITY's first frame were detected when using c_trace + USE_POLYGON.
Expanding and contracting the ENTITY's bounding box quickly and properly might work. Of course, the problem might not be fully understood here.
Derived from some version of some manual.
my.frame = n;
vec_for_min(my.min_x, my);
vec_for_max(my.max_x, my);
..and served up for no apparent reason.
