I'm pretty sure they said it would not increase prestige
No it doesn t, it just increases stats except prestiges.
I believe it does increase the prestige. My Hercules 5* with suicide mastery had 16,950 prestige max signature 200. After ascending him he bumped very close to 19,140. It is at my understanding that the prestige shouldn’t be increased, but it does. This only conclude for me that it does work with ascending for those extra boost stats which in turn should increase the prestige because your prestige goes go up a bit when you invest into adding signature stones on a champion.
Probably