The radiative linewidth of a two-level emitter (TLE) fundamentally limits the bandwidth available for quantum information processing. Despite its importance, no prior experiment has systematically examined how driving detuning affects the indistinguishability of photons scattered from a TLE—a parameter critical for photonic quantum computing. Here, we perform postselective two-photon interference measurements between mutually detuned resonance-fluorescence signals from an InAs quantum dot embedded in a micropillar cavity. At small mutual laser detunings ($\le 0.5\mathrm{GHz}$), the results are accurately described by the pure-state model [Wang et al., Nat. Commun. 16, 6453 (2025)], which treats all resonance-fluorescence photons as spontaneous emission with no well-defined emission time prior to detection. At larger detunings, we uncover an anomalous feature in the two-photon interference, where the normalized second-order correlation function under orthogonal polarizations yields $g_{\perp }^{\left(2\right)}\left(0\right)<0.5$.

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• Coherent control
• Light-matter interaction
• Optical coherence
• Photonics
• Quantum coherence & coherence measures
• Quantum optics
• Spontaneous emission