Current draw and voltage drop of DALI bus line
Using two 100 ohm resistors does not make a normal PSU into a DALI PSU. Either buy a DALI PSU using the list of certified products, or add an LM317 circuit to your PSU as mentioned in the linked question.
If you ever use a PSU with a current limit higher than 250mA with DALI equipment, as soon as a device tries to transmit it may damage the transmit transistor.
If your 15V supply is dropping to 7.8V after 200 ohms, it implies that your circuit is drawing 35mA. Unless specifically denoted as bus powered, DALI interface circuits must not draw more than 2mA per device under all conditions whilst not transmitting. So Infineon get away with 2x100R as current limiters for their special case of 2 devices on a bus but it is a terrible idea and totally non-compliant.
Application notes will have disclaimers about using them without doing your own calculations but manufacturers need to do better since they know that many beginners will assume that they are sufficient. Another common mistake on DALI app notes is to omit any rise time control components.