The attached document has an InputUser textfield (inserted manually via the
GUI). The textfield's XDependentTextField.getTextFieldMaster() method returns a
field master whose only connection to the text field seems to be that it lists
it in its DependentTextfields property. Other than that it does not seem to be a
real field master at all. It does not control the field's content nor does it
show up in the document's XTextFieldSupplier.getTextFieldMasters() collection.

The "real" fieldmaster can be obtained via
getByName("com.sun.star.text.fieldmaster.User.FOO"). Interestingly this field
master controls the actual content of the field but does not list it in its
DependentTextFields property.

To look at this yourself, open the attached test document, open the Basic IDE,
single-step to "End Sub" and then use the Watch window to examine the
"fakeFieldMaster" and "realFieldMaster" variables.