jmix 2.6.1
Создал свежий проект. В редакторе юзера у form
поставил labelsPosition="ASIDE"
и добавил поле <textField id="testField" label="test"/>
<formLayout id="form" dataContainer="userDc" labelsPosition="ASIDE">
<formItem>
<textField id="usernameField" property="username" readOnly="true"/>
</formItem>
<formItem>
<passwordField id="passwordField"
label="msg://com.company.jmix.test.entity/User.password"
required="true"
visible="false"/>
</formItem>
<formItem>
<passwordField id="confirmPasswordField"
label="msg://confirmPassword"
required="true"
visible="false"/>
</formItem>
<formItem>
<textField id="firstNameField" property="firstName"/>
</formItem>
<formItem>
<textField id="lastNameField" property="lastName"/>
</formItem>
<formItem>
<textField id="emailField" property="email"/>
</formItem>
<formItem>
<comboBox id="timeZoneField" property="timeZoneId" clearButtonVisible="true"/>
</formItem>
<formItem>
<checkbox id="activeField" property="active"/>
</formItem>
<formItem>
<textField id="testField" label="test"/>
</formItem>
</formLayout>
По итогу у testField
labelsPosition
сверху