Stove liners may become cracked after sustained burning at very high running temperatures, or if the liners become damp and are subjected to a high burn rate without a period of slower firing.
Assuming the liners are still in situ and able to support the throat plate correctly there is no need to replace them in the short term. It is not always necessary to replace a cracked liner, as it will still insulate the stove. However, once a liner begins to crumble, its insulation properties are reduced and this may cause irreversible damage to the stove body. At this point, they will need replacing.
Liners are consumable items and they cannot be replaced under warranty, per the user manual.