On Forge, coupled Minecarts disconnect and drop their Minecart Coupling when they are destroyed.

On Fabric, Minecart Couplings are not disconnected. This results in the remaining Minecart still having an attached Minecart Coupling connecting it to a Minecart which no longer exists. The most immediate consequence of this is that additional Minecart Couplings cannot be attached to the remaining coupled Minecart.