Not sure if this is working as intended, but in a match I just played the comp placed a Rocket/Groot countdown tile using their Team-Up AP. I then stunned the last remaining enemy when the countdown tile was at 1, but the it still went off.
Working as intended. It used to be that the countdown tiles were linked to one of the characters on the enemy team but they changed that so any countdown tiles work independent of them.
This change was kind of a double-edged sword. The behavior is now far more consistent and predictable (you can’t pause a TU countdown, rather than needing to stun an arbitrary member of the enemy team to pause it), but it makes TU countdowns a little more dangerous than others.