This happened in a VS AI game at about 4:15 (Lucio was level 5) and he activates summer anthem just as Thrall and Raynor are low, making them unkillable for 1.5 seconds. I have the replay file, but I can’t post a link to it, though I would be willing to send it some way. As a substitute, here is a recording of it happening: Lucio AI Summer Anthem - YouTube
Edit:
Didn’t realize there was protocol for sending replays. I have since sent it to the HeroesBugAttachments@blizzard.com email address, along with the proper info.