Hello. Sorry for this problem. Is this happening when opening an existing character file or happening every time (such as when creating a new character from the welcome page)?.
That's very strange indeed. We tried to reproduce this behavior in our test systems, but everything seems to be fine. Maybe you can try cleaning Mannequin's user preferences?
To do this, open C:\Users\[yourusername]\AppData\Roaming\Mannequin and then delete both user-preferences.json and user-preferences-dev.json (if this file also exists).
Let us know more if there's further problems.