That's what I thought it might be. Lots of reports of this on the Synology Forum:
https://community.synology.com/forum/2
I also use an RT2600AC and am holding off on the firmware "upgrade" until there is additional clarity on this issue.
[doublepost=1552039968][/doublepost]BTW, for anyone else reading this thread, I have been using the RT2600AC since it launched and it is a brilliant router. I don't want people who may be looking to upgrade their router being put off by this issue.