Hi,

I was told at a tournament today that it was ruled that you can't 'Train' a Rancor during Round 1 at Worlds. Something about there not actually being a "before" round stage at the start of a game.

Can anyone verify this and explain the reasoning?

Whoever said that is mistaken. The timing of Trained is "At the start of each round", which always exists, whichever round it is.

Also, "Before each round" would also exist for round 1. It happens just before the "start of round" trigger, after setup and all abilities triggered by setup have been resolved. (There's an implied "immediately" in timings that use before or after.)

Well that's one of the oddest mis-interpretations of a rule I've seen in a while....