I have not tested this exhaustively, but I had an issue today where I was trying to export what appeared to be a "vanilla" AutoCAD 2018 format file back to the 2013 file format, using AutoCAD Architecture 2018. On my initial attempts, I got a message saying there was an error and that I should try to RECOVER the exported file. I did so, and no errors were reported, but I got the dreaded future objects warning, and I could not add a Wall in the file. I had previously installed the Civil 3D object enabler on AutoCAD Architecture 2018, and suspected that was the reason why there were still future objects in the exported file. I uninstalled the object enabler, and was able to export to the 2013 file format without any errors, warnings or issues.
I am not certain if having the object enabler installed was adding Civil 3D objects, which the AutoCAD Architecture EXPORTOTAUTOCAD2013 command could not export/explode or if the enabler was causing an error that left future AutoCAD Architecture AEC objects in the file, but removing the enabler appears to have fixed things, for now.