I don't really understand that problem, exactly. I'm not aware of any restrictions for using AWS Identity Center (SSO) with an email address that happens to be a root email for another AWS account.
I checked the documentation but I couldn't find anything to show this to be a problem other than that the practice is discouraged.
Some tax forms like NY State fill-in forms have some scripted functionality that requires Adobe Acrobat to work, and they block rendering from other PDF programs. Even worse, once you've filled in the form, their PDF then blocks Acrobat from exporting to a normal PDF (the way you can save a print preview to a PDF for later) and so the only way you can get a real PDF of it is to print it directly and then scan it back to your computer.
I checked the documentation but I couldn't find anything to show this to be a problem other than that the practice is discouraged.