When using the artifacts tasty bone and wonderful wheat to summon the wolf and lama respectively. Intermittently (but frequently) every few minutes of gameplay and typically after being in combat one of my pets (either the wolf or the lama) stop moving. They remain rooted in place unless I summon them to my location using the artifact.
NB: Unequipping and reequipping the artifacts fixes the issue for a few minutes and the pets will behave as expected (e.g. following me and engaging in combat).