The control uses template Sitefinity\UserControls\Login\LoginControl.ascx. In the template the Text values are hardcoded. The text property of the control in the template are not exposed in the designer and this is whyyou cannot set them from the Login control properties.
1. You could edit the template and replace the values directly there or expose a public properties that will be set through the control designer.
2. You could remove the template and set the properties of the Login control as you have already tried. Note that in this control there are some problems with using the selectors for setting the property values ( for instance in DestinationPageUrl property) and this issue will be fixed in Sitefinity 3.7 SP4.
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