It's far too common for websites to redirect to some separate domain for sign in which isn't the one originally used to sign up, getting users used to "oh gotta copy the password again" as a totally normal thing that happens
Now on a few occasions I’ve had to copy passwords in order to access things in a different browser, and I think I did encounter one site some years ago where autofill didn’t work, but I really do find autofill almost completely reliable.
In Texas I've had more than one site where create the login on one site, but use that same login on multiple different domains that are NOT directly connected to a singular authentication site (id.me in the example).
You'll identify on id.me
People have just gotten used to this sort of thing unfortunately
For password safe users, auth being handled entirely on a different origin is completely fine, so long as the credentials are bound to (only used on, including initial registration) that origin. The hazard is only when login occurs via multiple domains—which in this case would mean if you had <input> elements on both tax.gov and id.me taking the same username and password, which I don’t believe you do. Your password safe won’t care if you started at https://tax.gov, the origin you created the credentials on was https://id.me, and so that’s the origin it will autofill for.