While you wait for an answer, I would also suggest that you try disabling any of your password software and try again - this could be a browser extension or, for example if using Chrome, this could be the Chrome/Google password manager feature. Your organization could have also deployed an enterprise-wide password manager utility or browser extension.
A quick test would be to open a private/incognito browser window, sign into Miro, and open a board’s Share settings, and then public access and see if the password field populates (incognito usually disables all browser extensions).
I am 99.999% certain that Miro would not be populating this field. Even if I set a password and come back, there is only an option to edit or remove the password - and when I edit, it doesn’t populate my current password. My personal password manager’s browser extension does see this field as a password field and would auto-populate it, if I hadn’t previously disabled that feature.