We are also having the same issue. We are using AC18 on a PC. We recently switched from AC15. I don't believe this project was started in 15 but I can't say sure if people were copy/pasting from 15. My coworkers say they didn't have this issue in 15 though. It's just begun happening with AC18.
The issue happens in the mirrored hotlink when I rotate the building 180 degrees and mirror in the hotlink manager. Our work around was to create a separate file of the plan mirrored and use both mod files. After searching the forums, this seems to have been an issue for AC12 and the recommendation for the work around was the same.
As KGrace said, there is a history of mirrored modules losing elements. When this happens, create a separate module of the mirrored version as K mentioned.
Erika
Architect, Consultant
MacBook Pro Retina, 15-inch Yosemite 2.8 GHz Intel Core i7 16 GB 1600 MHz DDR3
Mac OSX 10.11.1
AC5-18
Onuma System
"Implementing Successful Building Information Modeling"