The policy belongs to whoever is named on it
If the policy was in your roommate’s name and they move out, it leaves with them. Everybody remaining is uninsured from that moment, for belongings and liability both, and nothing announces it. In shared housing this is the most common way people end up without cover without choosing to.
Unrelated roommates usually need separate policies
Most carriers will not add unrelated adults to a single renters policy, and where they do, one claim affects both parties’ records. Separate policies are cleaner, cheap, and avoid an argument about whose television was whose.
Tell your insurer when the household changes
Who lives at the address is part of what the policy describes. A change is worth a phone call, which takes less time than discovering at a claim that the policy described a household that no longer exists.