High-frequency oscillations (HFOs) in modular multilevel converter based high-voltage direct current (MMC-HVDC) connected to AC networks have aroused concern in recent years. This paper reveals a new HFO phenomenon, where modular multilevel converter (MMC) exhibits impedance characteristics different from those in conventional HFO studies. To investigate this issue, a high-frequency dynamic model of MMC incorporating delay compensation is developed under different outer-loop control strategies. In addition, an AC line modeling method accounting for frequency-dependent parameters is proposed. Mechanism analysis demonstrates that the new HFO phenomenon arises from the adverse interaction between capacitive impedance of MMC and inductive impedance of AC network. Further, the causes of two types of capacitive regions in the high-frequency impedance of MMC are theoretically explained, which are linked to the relatively high proportional gains of current and power controls, as well as delay compensation. The effects of the frequency dependence of AC line parameters on HFO analysis are also demonstrated. Finally, the potential multiple HFOs and possible mitigation approaches are discussed for further study.