I've been going back and forth with support on this issue (see KXD-41286-339 for all of the issue. The Windows app master password will not change. It changes on 1password.com and the new master password is shown when revealed. I signed out of 1Password.com and signed back in again to 1Password.com with my new password, no problem. New password is shown as the master password when revealed.
Opened the app and signed on with original master password. The red banner appeared asking me to change the password which I did. The new password is now shown as the master password in the app. I then signed out of the app. I tried to sign back into the app with the new password. It does NOT work. Only way to get into the app is with original password. Looks like your developers have some more work to do.
Note that the windows app version is confirmed as the latest version.
1Password Version: 7.4.763
Extension Version: Not Provided
OS Version: Windows 10
Sync Type: Not Provided