Breakdown of Disorder-Suppressed Floquet Heating under Two-Frequency Driving

Periodic (Floquet) driving enables Hamiltonian engineering and nonequilibrium phases, but interacting systems eventually heat by absorbing energy from the drive. Disorder can greatly delay this process, yielding long-lived prethermal plateaus. Here, we show that this protection can fail when pulse-train control introduces a second driving frequency and when the disorder fluctuates. Using a natural-abundance ^{13}C nuclear-spin network in diamond, we observe sharp peaks in the late-time heating rate at the double- and triple-spin-flip resonance conditions predicted by bimodal Floquet interference and track their evolution with drive frequency. A switching-noise model attributes the resonant absorption to stochastic electron-spin dynamics that intermittently tune rare nuclear clusters into multiphoton resonance. Our results reveal a resonance-activated limit for disorder-stabilized Floquet phases and suggest new routes to dc-field quantum sensing based on an abrupt breakdown of prethermalization.