According to Japan's Ministry of Health, Labor and Welfare [1] it's a "substance that reacts to magnets", which "could be metal". Unfortunately, metallic contaminants have been found in many vaccines, so this would not be an uncommon quality control issue. [2]
This could lend some credence to the "COVID vaccine magnet challenge" that went viral in conspiracy theory circles. [3]
[1] https://asia.nikkei.com/Spotlight/Coronavirus/COVID-vaccines...