Any notion of "immutability" is not absolute. It refers to a restriction on the conditions under which the supposedly immutable thing could change.
For example in public blockchains like bitcoin, immutability can be undermined by a 51% attack, where the holders of a majority of the mining power can rewrite the chain's history.
In MultiChain, immutability is provided by the round-robin method, where permissioned "miners" sign blocks in rotation with each other. So immutability can be undermined if a majority of these entities collectively decide to rewrite the chain's history.
There is no such thing as absolute immutability in computer systems, until data is moved to a non-rewritable storage device.