I used WAU to update this afternoon, and now all event names are displaying in my second chat frame. (I normally use that chat frame for guild, party, and raid chat display.)
The only mods that auto-updated were libraries. Did the author of one of these libraries leave some debug code in that was printing event names to the second chat frame?
Unfortunately, I cannot individually disable these libraries without losing all functionality of every mod I use, so I'm unable to determine which of the updated libraries may be causing this.
Ace2 - Changes from 44693 to 44816
Babble-2.2 - Changes from 44730 to 44796
Thanks for helping to investigate this!
Edit: I indicated earlier that DewdropLib might be to blame, but I was able to "disable" it by completely removing its folder from Interface\Addons. (Even though I had disabled it in the Addons configuration screen after login, other addons were still loading it until I removed it completely.)