Unexpected repeating pixels

Hi all.
I’m having a strange problem with two strings of LEDs.
I bought a 200 LED string of these for my Christmas Tree. Cut of the basic controller they came with and connected them to a D1mini flashed with WLED. They worked perfectly after changing the colour order and the tree looked brilliant.
The only problem was that the 200 LEDs were a little short for my 7ft tree and the loops of lights were a little far apart. So I went back to the same listing and bought a string of 100 LEDS. Again cut off the controller and connected the new string onto the end of the old string.
I should say that the 200 long string turned up as two bundles of 100 soldered together. There are also other solder joins along the length of the string.
I expected pixel 1 of the second string to become pixel 201 of the new bigger string and to just have to adjust the total number of pixels in the settings.
But pixel 1 (physically number 201) of the new string is mirroring pixel 1, It’s like they’re in parallel rather than in series.
I tried playing around with settings and segments but no joy.
I’ve tried flashing a very basic Arduino Pixel test sketch that just lights the first three LEDs in red, green and blue respectively. This caused 1-3, 101-103 and 201- 203 to light up.
I then just put wled back on it and set the pixel count to 300 and its doing the same - 1-200 are fine, 201+ mirrors pixel 1 onwards
I have no idea what the actual chips are, the listing claims WS2812B and SK6812. Are there pixels that need to be assigned an unique identifier before use?
I’m really confused!!

Some lights have been found to act like that. No way around it AFAIK.