It was not clear when the photo of Jimin allegedly wearing the shirt was taken, but local news outlets reported the image was from 2013.
BTS later apologised to its fans for not being able to make an appearance. A generation of Koreans suffered under Japanese occupation, while hundreds of thousands of Japanese died when the USA dropped bombs on Hiroshima and Nagasaki, leading to Japan's surrender.
The shirt, which is still available for sale online, bears the slogan "Patriotism Our History Liberation Korea" and features an iconic photo of the atomic bomb dropped on a Japanese city.
BTS member Jimin reportedly wore the shirt a year ago, on August 15, when Koreans celebrate the end of Japanese occupation in 1945.
A BTS Japan fan site also announced their appearance had been postponed, along with an apology to fans who had been looking forward to the broadcast.
"I'm from Hiroshima. I can not accept BTS appearing on Music Station", said one Twitter user.
Back on November 8, TV Asahi's 'Music Station', abruptly "postponed" BTS's scheduled appearance on the popular music program, forcing Big Hit Entertainment to cancel BTS's flight to Japan on the same day and disappointing a huge crowd of fans who gathered at the airport to see the boys.
"A-bomb t-shirt" was one of the top trending topics on Twitter Friday morning. "Jimin don't blame yourself", said one Twitter user.
The cancellation comes as ties between Japan and Korea have becoming increasingly strained in recent years.
However, Tokyo maintains the issue of compensation has been settled under an agreement attached to a treaty that was signed in 1965, calling the ruling "unbelievable".