Did you press enter in the textbox?
Eg. If you had the anchor set to bottom left, and then pressed enter in the textbox, you'd be inserting one line of nothing into your textmesh, pushing it above origin. Then if you replaced the text with something that didn't contain a newline the text would appear to move down. Make sure you've got the correct anchor set as well.
You should have the correct number of characters in the textmesh - to avoid reallocations, the mesh will not resize its buffers internally if you try to insert more characters than has been allocated.