I encountered a bug tonight in Competitive 3v3 around 1 a.m. CT. Central U.S. PC.
I was queued as Lucio in the 3v3 match as the timer was ticking down. At the last second (quite literally), I thought I might change to Mercy. I had selected Mercy but when I queued into the game I was still Lucio. NBD. I played the round. We won.
Then, when the hero select screen appeared again, Mercy was pre-selected. I clicked the “Change Hero” button and nothing occurred. The button did not work and I was not able to change heroes. I joined the next round as Mercy. We won again and on next hero select screen it was back to normal.