Related to bugs #144409 and #317746 reported to specific applications that uses the password input widget, this seems to be a general issue with the widget. Indeed, I could confirm this is a broader problem because it is also present on KDE Wallet Manager password request dialog.

As described in bug #317746, the problem is:
"[in password input boxes], typing the "^" key before the letter "o" to form the ô character will actually generate two characters in the text input box, possibly the two independent characters "^o" instead of correct ô. I.e. dead keys seems disabled for KDE password entry boxes."

As a workaround, it is possible to type the password correctly somewhere else and then paste it into a password input box, then it will work as it should.

This can also be an issue with QT itself, but I can't confirm.

Reproducible: Always

Steps to Reproduce:
1. Configure you keyboard to use Deadkeys (if it is not alredy configured).
2. Open a KDE application that requests a password (kdesu or KDE Wallet, for instance)
3. Try to type a letter with accent, like ^ followed by o
Actual Results:  
Two independent characters are output into the password, the accent, then the letter, dead keys are ignored.

Expected Results:  
A single character should be input into the passord, the accented letter (hidden by the placeholder character, but still one).

Typing ^ then o on any text input widget, when dead keys are enabled, will lead to the correct ô character.