A bit more info may help direct the investigation:
1. Is it a large or small effect? (A small effect may simply be due to rounding/analysis errors and not truly a physical effect)
2. is it happening at all temperatures, or only in a range? How does that range 9if that's the case0 compare to, say, engine flat rating temperature?
3. (IT Help desk "is it switched on?" question
) is anything else varying at the same time, other than DIRECTLY due to the mass (config, thrust, etc.). I assume not, but "never assume"