This is physical, since there is a gap between the thin film and the PML. This means you are simulating a cavity formed by the thin film. Please make sure the thin film has substrate or not. Even with substrate, under proper conditions, such "oscillation" can happen when there is resonance.
When there are air gaps the cavity ends are in air/background material. When there is a substrate, one end of the cavity is in the substrate whereas the other one is still in air. Anything that forms two materail interfaces potentially can have resonance in broader spectrum, when it fulfils optical-path-difference is integer number of wavelength.
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