Parallelism causes fill() to attach to wrong locator

While you show some code, can't tell the overall structure for your app, setup, this global and and shared? My guess what you are getting is expected as you have 'n' thread all accessing shared data without any regards to anything...

Is process.env.UITestEnvironment always defined? process.env.UITestEnvironment || process.env.UITestEnvironmentDashboard indicates that you expect the env variable to be undefined in some cases

then the first goto would goto(undefined) ?

Yes they are always defined.

I mean the entire setup project is literally the same. When I run it fully parallelized, I get 3 workers, each running one of these commands once, as expected.

import { test as setup } from '@playwright/test';
import * as dotenv from 'dotenv'
dotenv.config()

setup.describe('User Session Capture', () => {

    setup('authenticate as site director', async ({ page }) => {
        //login as site director
        await page.goto(process.env.UITestEnvironment);
        await page.getByPlaceholder('Username', { exact: true }).fill(process.env.userNameSiteDir);
        await page.getByPlaceholder('Password', { exact: true }).fill(process.env.passwordSiteDir);
        await page.getByPlaceholder('Company Number', { exact: true }).fill(process.env.companyId);
        await page.getByRole('link', { name: 'Sign In' }).click();
        //waiting for next page to load
        await page.waitForLoadState('networkidle');

        //sending to dashboard to save state
        await page.waitForLoadState('networkidle')
        await page.goto(process.env.UITestEnvironment);
        await page.waitForURL(process.env.UITestEnvironment || process.env.UITestEnvironmentDashboard);
        await page.context().storageState({ path: process.env.siteDirectorLogin });
    });

    setup('authenticate as admin', async ({ page }) => {
        await page.goto(process.env.UITestEnvironment);
        await page.getByPlaceholder('Username', { exact: true }).fill(process.env.usernameAdmin1);
        await page.getByPlaceholder('Password', { exact: true }).fill(process.env.passwordAdmin1);
        await page.getByPlaceholder('Company Number', { exact: true }).fill(process.env.companyId);
        await page.getByRole('link', { name: 'Sign In' }).click();
        //waiting for next page to load
        await page.waitForLoadState('networkidle');

        //sending to dashboard to save state
        await page.waitForLoadState('networkidle')
        await page.goto(process.env.UITestEnvironment);
        await page.waitForURL(process.env.UITestEnvironment || process.env.UITestEnvironmentDashboard);
        await page.context().storageState({ path: process.env.admin1Login });
    });

    setup('authenticate as enduser', async ({ page }) => {
        await page.goto(process.env.UITestEnvironment);
        await page.getByPlaceholder('Username', { exact: true }).fill(process.env.usernameEndUser1);
        await page.getByPlaceholder('Password', { exact: true }).fill(process.env.passwordEndUser1);
        await page.getByPlaceholder('Company Number', { exact: true }).fill(process.env.companyId);
        await page.getByRole('link', { name: 'Sign In' }).click();
        //waiting for next page to load
        await page.waitForLoadState('networkidle');

        //sending to dashboard to save state
        await page.waitForLoadState('networkidle')
        await page.goto(process.env.UITestEnvironment);
        await page.waitForURL(process.env.UITestEnvironment || process.env.UITestEnvironmentDashboard);
        await page.context().storageState({ path: process.env.eu1Login });
    })
});```

What follows is just a wild assumption of me, not knowing how your app operates. Could it be the case, that the page when filling the username initially hasn't finished loading. And after switching to fill the password there is something js like on the page that switches the active focused input to be username?

Oh, thats actually a good idea