My Steam account is not linking my characters

You should be able to choose which sub-account you're using at the top left of the page, for any purchases you wish to make. You can also make purchases in-game if you wish, to ensure you're making your purchase on the correct account. I hope this helps!

Please note that currently it is not possible to see Path of Exile 2 characters on the website, so you'd need to check your account name and four-digit discriminator instead.