I don't have a V60, but my bonavita drip machine has a clear plastic window that has tiny cracks like that, but only exactly where the hot steam escapes between the dripper and the shower head, so I suspect we both have heat fractures from thermal shock. For me at least, it still is watertight so I'm not worried about it too much. Yours looks like it might be soon losing structural integrity.
I would stop preheating with boiling water straight from cold, can you first heat it with hot tap water for 30 seconds or so? Might reduce the thermal shocks if it's already at 120 instead of room temp
