on cold start is not a well defined period , so i can't do any more than guess that maybe your bov doesnt like cold temps much. i swapped mine to a GFB not long after buying.
Turbo flutter is the result of compressor surge as the compressed air comes back through the spinning turbo when your throttle closes. On cars with a vac triggered bov then you normally get a rush rather than flutter so it could indicate your bov is staying closed. Perhaps this is your e-bov not opening until the engine warms some.
or it might be something completely different, IDK.