I have resolved the issue for myself... The culprit was how I had initially saved the login credentials in LastPass.
Apparently I must have used the option to capture & save ALL field values which included a hidden Captcha response and other fields which may still be used to login, as can be seen in the following screenshot:
I tried deleting all but the minimum required form fields, but that didn't work. What fixed it for me was completely deleting the LastPass entry for the SmartBear community, then logging in using a fresh browser tab and saving the credentials as a new entry. I then confirmed that it continued working in the same and other browser sessions.
The reason Edge was working fine is because it doesn't (yet) officially support LastPass and so the hidden field values weren't being modified by LastPass when the page loaded.
You got me thinking about LastPass. While my ultimate issue was different than yours it was related to LastPass. For my saved site I had the URL pointed to https://community.smartbear.com/. Because I also have a seperate AlertSite login I should have saved the URL for the forum to the login screeen URL instead. Once I updated the site URL to the login page it works fine now.