Amber Heard: Miraculous Weight Gain?

Amber Heard arrived back in L.A. after getting fitted for her "Justice League" costume in London ... exactly one week after the fitting was supposedly cancelled because she was 20 lbs underweight.

TMZ broke the story ... Johnny Depp's team wanted to take Amber's deposition a week ago, but her lawyers said she was unavailable because she was going to London for the movie fitting.

We found out Amber never went, and her people said the fitting had been cancelled because she had been so stressed out she lost 20 lbs ... so the fitting was scrapped.

Our Johnny sources are scoffing ... they never believed she was supposed to be in London a week ago and that the story was concocted so she wouldn't have to be cross-examined under oath about the alleged beating.

Fact is ... she looks thin in the video.