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Identifying early markers for autism spectrum disorder (ASD) is a clinical priority. This study investigated interpersonal affect synchrony (IAS) as a measure of interactional quality in a longitudinal cohort of 90 high-risk infants. We aimed to disentangle its contribution from mutual gaze and identify data-driven social interaction profiles linked to sensory traits. Parent-infant interactions were recorded at 6-18 months; IAS was quantified using Cross-Recurrence Quantification Analysis, and ASD outcomes were determined at 18-24 months. Infants later diagnosed with ASD (n = 25) showed significantly lower IAS (F(1,84) = 5.89, p FDR = 0.023) and synchrony stability (F(1,84) = 5.37, p FDR = 0.023) than non-diagnosed infants (n = 65), yet the groups did not differ in mutual gaze (p = 0.200). Logistic regression analysis further showed that IAS (OR = 0.561, p FDR = 0.038) and synchrony stability (OR = 0.013, p FDR = 0.038) both significantly predict clinical outcome. K-means clustering revealed three profiles: "High Gaze-High Synchrony," "Mid Gaze-Low Synchrony," and "Low Gaze-High Synchrony." The "Mid Gaze-Low Synchrony" profile was significantly associated with a later ASD diagnosis (p adj = 0.031), while the "Low Gaze-High Synchrony" profile was linked to higher sensation-seeking traits (p adj = 0.028). The quality of parent-infant affective connection is a more robust early marker for ASD than the quantity of mutual gaze. These findings reveal critical heterogeneity, identifying a high-risk "gaze without engagement" pattern and a potential adaptive pathway to synchrony, underscoring the need for individualized strategies in early screening and intervention.