I cast Borys (Scarlet Spider) using enweb (web-slinging), returning Cren of the Spider Brood (the back face of Cren, Undercity Dreamer / Miles Morales). Cren was my opponent's commander, which I controlled at the time. Borys received only 2 +1/+1 counters. It should have received 6 because the returned creature left the battlefield (and even went to a hidden zone, not that this should matter), therefore the game should use last known information about it to determine its mana value. And Cren/Miles is a modal double-faced card, as shown by the single and double triangle symbols, which has a mana cost on its back face, so while it was on the battlefield it had mana value 6.