VNA Tuning in Enclosure -- LiPo Battery Detuning Antenna
This sounds not just possible but likely, especially once you are working with decently high frequencies. This is also a common problem with antennas where their center frequency will shift (sometimes so much it no longer covers the band of interest).
Depending on the material, it could be the plastic being very lossy (PVC is for instance very lossy at certain frequencies due the it being polarized) or the fact that your plastic is a dielectric that changes the impedance of your lines.