I think they're both related to upholding territorial claims, which the PRC says is a "core interest" (ie, something they're willing to go to war over), but I don't think it's viewed as a direct domino effect.

The more direct link is potentially to the CCP's claim to legitimacy as the government of China. If they can't defend China's "historic" claims to territory then they could lose legitimacy in the eyes of an increasingly nationalistic public.