I don't see what rated or unrated games have to do with it.
Both players testing have the addon enabled.. since it relies on communication it would be pointless to test with people that don't have the addon installed or not sharing skills.
The addon doesn't use any conditionals to run different code in different environments.
It registers its prefix and the GROUP distribution and sends over the GROUP distribution when it needs to communicate information (skill duration in particular).
It then relies on AceComm-2.0 to handle said group environment and select the appropriate channel.
The same 2 people testing it in a party outside or in a party/raid in pve instance have no problem to get it to work.
It does not work for them when they do arenas.
Since the code that runs (addon side) is the same in all environments
function myAddon:OnCommReceive(pre, sender, dis, cmd, ...)
I believe it's a problem with GROUP distribution not working properly in the case of Arenas.
I don't have an arena team to test this myself,
so I was hoping that one of the authors on this board that has experience with/writes addons for Arenas
would be able to verify which of the base SendAddonMessage types ("PARTY", "RAID" or "BATTLEGROUND") should be used in that environment,
and AceComm-2.0 modified accordingly.
If none has that info I understand how you don't want to be making changes to the library based on my conjecture,
but "probably an issue with your addon" doesn't really help either :)
Why couldn't you think of that and tested that before the Ace2 commit a few days ago? If this actually is an issue, this will cause another gigantic repacking on the SVN for like 2 hours. The SVN is already slow enough as it is. >(