Yes, that's what I meant!
The way the math is working is... It takes the sprite sheet and uses the columns/rows setting to know how many boxes to slice the texture into. So, if there's 1 row and 3 columns and you want to read index 1 (the second image), the UV used would be... (0.3333,0), (0.66667,1)
...I think that might be it. Can you try making it so your texture is 4 units long instead of 3? (So the UVs would land on multiples of 0.25 instead of 0.3333 repeating) The UV math's float rounding might be what's pushing it over the edge? I think I might also be able to fix this by taking the far right UV (0.66667 in that example) and subtracting a very small number like 0.0001 from it. I'm away from my work computer atm, but I'll figure out exactly what line that would need to go on in just a bit!