The Reds were awarded the spot-kick inside the opening ten minutes at The Hawthorns when Thomas and Luis Suarez tangled on the edge of the penalty area with Charlie Adam converting in the match that eventually ended 2-0 to the visitors.
But Thomas believes the call from Mason's assistant was an error and feels it should have never have been given.
"It didn't think it was a penalty at the time and I don't even think the referee did," he told the Birmingham Mail.
"It didn't look like he was going to give it and then the linesman flagged and I guess that's what made up his mind.
"I always thought the assistants were exactly that. I thought they were there to assist if the referee couldn't see it.
"Not only was the referee close enough but he seemed to have seen it and didn't think it was a penalty."