After copy, the bit rate mode was changed from Variable to Constant.
Is there possible to copy with no changes? (Include tags)
In copy mode, audio bitstream is never changed. Some packets may be dropped when necessary to maintain A/V sync (especially at the stream start and at the end). For any in-depth answer, please provide more than zero details*.
You should also consider updating to the latest nightly, the 2.7.8 release has got long in the tooth.
*) Properties of the source file as listed by MediaInfo, properties of the output file as listed by MediaInfo, admlog.txt from loading and processing the source video.