Ceramic Capacitors v's Tantalem Capacitors

[ppppenguin]

Sorry to be picky about measurement technique but here goes. Presumably you're using the oscillator frequency as a proxy for the capacitance. When you're heating/cooling the cap it's all too easy to be heating or cooling other components which can affect the frequency. The mica cap may have a tempco which is offsetting the tempco of something else.

Or of course the so-called NPO caps may be nothing of the kind.

Not sure which caps you're using The only 33p 1kV ceramics I can find that meet your description are these: https://uk.rs-online.com/web/p/ceramic-s...s/8312907/ These are N750 , not NPO.