Much like @Marc_Carlyon has mentioned, it’s most likely going to be the size of the text box you are outputting the concatenated string to.
I would imagine what is happening is that when a number is added to the text, in some cases it fits the UI Text box you have placed, but in some other cases it doesn’t, so it then wraps a digit, or more around to the next line. You obviously can’t see the next line because the height of the UI Text box isn’t tall enough.
The suggestion would be to extend the right hand edge of the UI Text box a little further so that the value fits, or, create a separate UI Text box for the static text, and have a UI Text box just for the guess, you can then make sure the box is plenty large enough and perhaps right-align the text so that it also looks nice.
Hope this helps.