Problem is, now it appears that the data wire to the LEDs is running separately from the ground wire, particularly from the end of the first strip to the start of the second.
It is substantially more important that the data wire always travels paired with (bound to) the ground wire, wherever it goes.
Similarly, the 5 V power and the ground must travel everywhere as a pair. This is actually more important in this respect than a "star" grounding pattern.
The 1 mF capacitor and 470 Ohm resistor should be mounted at the start of the LED strip.