Sounds like a slight error caused by the upscaling algorithm. Upscaling is done by the program making a guess to fill in the new pixels created by the larger size, and sometimes the guess isn't great.
There's a few different things you can try to get around it, but I'm not sure which one would be better for your particular problem. You can try a different program, a different upscaling algorithm, upscaling by only one tile at a time, taking out the error and filling in the gap by copying a pixel from the side or another upscaling attempt, or just drawing it in with your own best guess.
I'm not able to make an upscale of my own right now, but I'll try to remember to make one in a few weeks when I will hopefully be free.