Black plastic :(

[OneOfEleven]

Be very careful when trying to mix RF and black plastic (best never to do so).

Black plastic can contain carbon and/or graphite for part of the colouring.

[cernohorsky]

There's definitely something to this. When I made the first prototype, the results differed significantly from the calculated values, and I thought it was simply additional capacitance from the filament material. But this could really be the cause. Unfortunately, I never tried it with a different color or other filaments.
In any case, thank you for the comment. This will also help in future projects.