ACEScc is supposed to be an internal working format for grading systems. It is not intended to be written to files.
While ACEScc image data written to a 16 bit integer format should in theory preserve the values from the ACEScg original, somebody sending files as ACEScc would give me concerns that the people doing the VFX pulls don’t really understand ACES. You would certainly not want to use 10-bit DPX for ACEScc as 1024 steps is not enough for the range that ACEScc codes.
Also it is concerning that you are getting negatives from the ACEScc source. ACEScc is a pure log coding, and should not be able to code negatives at all. Sorry misread “below 1” as “below 0”. ACEScc is ranged 0-1, but once converted to ACEScg should normally include values above 1, as ACES is linear unclamped float.
As Alex says below, if when converted there is nothing above 1.0, that suggests clipping may have occurred. Are you using an OCIO to convert, either in the read node or in an OCIOColorSpace node?