I doubt it is cineform that is guilty in this case. What software was used to do the encode to x265? Because you said it is encoded with DCI-P3 I assume there was (different) color conversion. How it is possible:
1) magicyuv+avi has no metadata at all -> everything default.
2) cineform+avi has some metadata -> replaces defaults (not same).
Just guessing.
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