In the examples I have attached. I am merging at centre those 3 vertices, and when I try to do a loop cut it stops at the loop cut, and does not go to the tip of my mesh. But If I grab the same vertices and scale to 0, they merge, at centre of the middle vertex, and when I perform loop cut it does indeed go all around. So I'd like to understand what does it have a different result, scaling to 0, and merging to centre.
Thank you.
Scaling to 0 does not affect the underlying topology of your mesh, meaning that you still have the original 3 points, and all your faces are still quads, in your mesh. So the loop cut can be completed.
However, when you merge vertices, this changes the topology and effectively creates only 1 vertex out of the 3 original, thus creating triangular faces that cause the loop cut to stop when it hits a triangle.
Are you sure they've actually merged when you scale to 0? I think you'll find that you still have 3 vertices, albeit on top of each other. To check, you can box select the area where they've apparently merged and use Merge by distance. In the bottom bar you should (briefly) see the number of vertices that have been merged. I suspect that's why you're seeing different results when you try to do a loop cut. It might well give you problems later too if you have 3 vertices on top of each other!
