I think, that value is always stored in MC. You can only put your bitcoins into "escrow" (lock them in main-chain) and use "value" on side-chain.
Shouldn't you then believe that the severing of the 2-way peg would result in a scBTC value of zero in all cases? I think I've convincingly shown that this would not always be the case. Therefore, a % of the value must actually be stored on the sidechain's ledger.