It gives some scrambled text as it is not send as UTF-16 text but UTF-8 text.

Create a new test project and add this view as an example:
Use Windows.pkg
Use DFClient.pkg

Deferred_View Activate_oTestToolTip for ;
Object oTestToolTip is a dbView

    Set Border_Style to Border_Thick
    Set Size to 200 300
    Set Location to 2 2
    Set Label to "TestToolTip"

    Set psTitle of ghoToolTipController to "Test title"

    Object oForm1 is a Form
        Set Size to 13 100
        Set Location to 13 101
        Set psToolTip to "Test tool tip text"

The error shows like this:
Name:  Tooltip1.png
Views: 78
Size:  20.3 KB

To fix for now you can subclass the cToolTipController and fix the method your self:
// Replace the standard oToolTipController in your src file with an object of this class.

Class cToolTipControllerFix is a cToolTipController

    // Eksample of a corrected method that takes a native utf-8 string and send as UTF-16 text message.
    Procedure Set psTitle String sTitle
        Integer iVoid iIcon
        Pointer lpsTitle
        Handle hToolTip
        String sIcon
        WString wsTitle
        Set psTitle_private to sTitle
        Move sTitle to wsTitle
        // Icon & title must be set together. Determine what the icon is so we can set it.
        // It will be either an icon ID for a standard icon, or an icon filename....
        Get piIcon to iIcon
        If (iIcon <> TTI_NONE) Begin
            Set piIcon to iIcon    // this will set the icon & title.
        // Test if an icon filename is set....
        Get psIcon_private to sIcon
        If (sIcon <> "") Begin
            Send LoadIcon sIcon   // this will set icon & title.
        // If we got here then there is no icon, just set the title then....
        Get phToolTip to hToolTip
        Move (SendMessage(hToolTip, TTM_SETTITLE, TTI_NONE, AddressOf(wsTitle))) to iVoid

Object oToolTipControllerFix is a cToolTipControllerFix
    Move Self to ghoToolTipController
The result:
Name:  Tooltip2.png
Views: 89
Size:  18.2 KB