This happened to me and it stumped me for a while til I realised that the LED strips I was using had an arrow on them. Reversing the renegade strip worked. So if you ask yourself why two LED strips you attach together even with 5V and GND matching up, that each work, but together they don’t…
…check that the orientation of the arrows match.
The other dead giveaway is the labeling on either side of the cut points.
The left is marked “DO” (Data Out) while the right is “DIn” (Data In).
That’s pretty standard across many of the addressable LED types.
Glad you got it working in the end, now the fun begins!
Ah ha, I was not being totally observant @divsys . Indeed, now I look closely, both the strip types I’ve got IP30 and IP67 have got DI/DO or DIN/DO on them with matching arrow directions.