Could you please be more specific? How do you know?

The flow I've been using goes like this, starting at the IRS website:

1) irs.gov homepage -> click 'Sign in to Your Account'

2) https://www.irs.gov/payments/your-online-account -> click 'Sign in to your Online Account'

3) https://sa.www4.irs.gov/secureaccess/ui/?TYPE=blahblah (uuid/session url param)

I'd consider using login.gov, but apparently someone didn't finish the job yet even though it's been at least one year since this was announced.

It occurs to me SSO is nice in theory but one might surmise the diffusion of responsibility is problematic.