Well, or you could learn the rules of the game. Because this is not a bug. It's the game handling the rules correctly.

You allowed Ulamog to attack, so the annihilator trigger went on the stack. And exists there independently of its source. It doesn't matter what happens to the source after the trigger made it to the stack. If the resolving trigger needs information about the source (like how many counters were on it), and the source doesn't exist anymore, then the game uses last know information of the source. And the source can even perform actions if instructed to, despite not existing anymore.

608.2h If an effect requires information from the game (such as the number of creatures on the
battlefield), the answer is determined only once, when the effect is applied. If the effect requires
information from a specific object, including the source of the ability itself, the effect uses the
current information of that object if it’s in the public zone it was expected to be in; if it’s no
longer in that zone, or if the effect has moved it from a public zone to a hidden zone, the effect
uses the object’s last known information. See rule 113.7a. If an ability states that an object does
something, it’s the object as it exists—or as it most recently existed—that does it, not the
ability.

OF COURSE, i expect a compensation for this bug, it was a ranked match