That’s your problem then. It’s just going to hit the edge of your UVs and clip through them as it loops around. Ribbons also don’t behave like regular particles. Ribbons are a series of connected sprites, and each slice acts as an individual particle. So when you try to scale or adjust color/alpha over life with a ribbon, it won’t adjust the ribbon as a whole, but each individual slice. If you scale its alpha over life, the end of the trail will fade out as each slice is going through its own individual lifetime.
So you have three options with this to keep that edge from clipping.
A) Multiply your alpha by a gradient so it fades out before it gets to that edge
B) Adjust your ribbon alpha over life so that it fades to black both at the beginning and end of its lifetime
C) Don’t pan your texture. Ribbons are meant to be attached to something moving, so your texture for a ribbon will either be something that seamlessly tiles, or the overall shape stays static.
Ribbons in Unreal are also pretty flawed and act weird, so it’s sometime difficult to use them.