Time on clock added out of nothing

[alexispiazza]

Exact URL where the bug happened
https://lichess.org/bt6UNSyL
https://lichess.org/forum/lichess-feedback/bug-on-bullet-game-time-added-for-opponent
Steps to reproduce the bug
Can't reproduce. Time was added to opponent during a bullet game.
What did you expect to happen?
No time added.
What happened instead?
Time was added.
Operating system and browser version
Windows 10, Chrome.

In the links above you can see what happened on move #70.

I presume that this was a bug, but it could be a cheat as well. Around 14 seconds was added to my opponent when he had only 2 seconds left on his clock.

[randompearl-stack]

See Game (ply 2 - 3) and #2345
In all three games the time changes are very close to multiples of 15s, which is the time given every click on the add time button.