Google reviews Wear OS apps against additional requirements beyond the base Android policy: the watch app has to function meaningfully on its own (not require the phone app open to work), respect battery/complication guidelines, and follow Wear-specific navigation patterns.
Frequently caused by a companion watch app that's effectively non-functional unless the paired phone app is actively open, which Wear OS review treats as not meeting the standalone functionality bar.
"Your Wear OS app does not function independently of its companion phone app, which does not meet the Wear OS compliance requirements for standalone functionality."