It's the same in incognito mode, so it seems it's not on the browsers' side.

@ovasileva I cannot replicate this. I used a test account I haven't used in over a year and after logging in I get the new Vector. The code also supports this.

My guess is one of the accounts you've been testing on changed another preference. Note any change to any preference will have cached the previous value of legacy. Note changing a preference doesn't necessarily mean a visit to Special:Preferences, for instanceif you used the sidebar even during the testing phase with that account, you would have triggered an update to preferences and marked yourself in the legacy group.

Here's what I saw post login with that account:

Suggested: close as invalid

@ovasileva: Can you confirm that this is still happening on the account that you were using?

@Jdlrobson: I wondered out loud to Olga if this was related to T258971: PrefUpdate event.isDefault field is not recoded correctly for some user properties. Having now looked into that task, I don't believe that's the case.

@phuedx, @Jdlrobson - difficult to confirm on wikis that have already been switched, but can confirm on frwiki later today.

I can't reproduce this. Here's what I've tried:

In production

1. I logged into officewiki with my Phuedx (WMF) account, which was created before 22nd July 2020
2. I observed that I saw the new Vector treatment
3. I navigated to Special:Preferences
4. I ran the following in my browser's console

mw.user.options.values.VectorSkinVersion // => undefined

4. I updated my date format preference
5. I clicked "Save"
6. I observed that I still saw the new Vector treatment
7. I ran the following in my browser's console

mw.user.options.values.VectorSkinVersion // => '2'

In development

1. I created a new development wiki with MediaWiki-Docker
2. I observed that I saw the old Vector treatment
3. I created a new user, Phuedx
4. I deleted any user preferences in the user_properties table
5. I added the following to LocalSettings.php

$wgVectorDefaultSkinVersion = $wgVectorDefaultSkinVersionForExistingAccounts = $wgVectorDefaultSkinVersionForNewAccounts = '2';

6. I observed that I saw the new Vector treatment
7. I navigated to Special:Preferences
8. I ran the following in my browser's console

mw.user.options.values.VectorSkinVersion // => undefined

7. I updated my date format preference
8. I clicked "Save"
9. I observed that I still saw the new Vector treatment
10. I ran the following in my browser console

mw.user.options.values.VectorSkinVersion // => '2'

Further to the above, you can reproduce the initial conditions for this bug report on any of the desktop-improvements wikis by running the following in your console:

new mw.Api().saveOption( 'VectorSkinVersion', null );

@ovasileva: If there's anything missing from my investigation above, I'd be happy to add it. However, I can't reproduce this in a production or in a development environment. My recommendation would be to resolve this task as Declined.