The alpha over node functions incorrectly currently.

It should be possible to have an RGB value with zero alpha and result in an addition as the over operation should be:

Over.RGB + (Background.RGB * ( 1 - Over.Alpha) )

Currently, the code does a check for a less than or equal and does a direct copy at line 50:

static void do_alphaover_premul(bNode *UNUSED(node), float *out, float *src, float *over, float *fac)
{

if(over[3]<=0.0f) {
copy_v4_v4(out, src);
}

[...]

This is incorrect behavior and results in a broken composite if alpha is zero whereby a copy happens. The correct solution would be to change the sign to less than, or do away with it altogether and clamp the alpha to a zero value and permit the add of the values as opposed to copy:

if(over[3]<0.0f) {
copy_v4_v4(out, src);
}

NOTE: I've added a Color Ramp to clamp the output from the renderer as it is delivering negative alpha values and confusing the issue.