Hi @Joshua,
Thank you so much for getting back to me and for sharing the results of your testing. I’ve just returned to my workday today and really appreciate the time and effort you've put into investigating this.
For reference, my Teams version is

, and I can confirm that when I sign in using "No, this app only" and then sign out, my previously used account still appears under "Pick an account".

Based on what you've shared so far, I'm noticing several different scenarios and some inconsistent behavior across environments, which makes it difficult for us to draw a definitive conclusion from our position in the community.
Since you're using a work account, could you also confirm whether it's a end-user account or an administrator account?
If you're using an administrator account, I would recommend opening a Microsoft Support ticket. This would allow the Microsoft engineering team to investigate the behavior more thoroughly, review any relevant diagnostics, and provide an official determination as to whether this is an intended change, a security/privacy enhancement, or an issue that requires further investigation.

If you're an end user, I'd kindly suggest reaching out to your Microsoft 365 administrator and asking them to open a support ticket with Microsoft on your behalf.
Tips: You can switch off the Support Assistant as shown in the screenshot, then type a few short characters into the search box and select Get Help. After that, wait for the Contact Support button to appear and click it. At the final step, be sure to clearly describe your request in the description fields so the support engineers can quickly understand your issue and assist you more effectively.
Also, if you decide to go down that route and receive any new information or an official response from Microsoft, I'd highly appreciate it if you could come back and share it with me and other community members who may have the same concern.
I believe that would be one of the quickest ways for all of us to gain a clearer understanding of whether this behavior is expected, the result of a recent change, or something else entirely.
Your findings could be extremely valuable to others who are following this discussion and may be experiencing the same behavior in their own environments. Thank you again for taking the time to investigate and share your observations