Text boxes keep on moving in Unity

You've configured your text boxes to position themselves relative to the center of their parent element, not relative to the bottom edge/corner:

You've also configured your canvas to position elements according to exact pixel sizes (so the size of the UI elements and their distances apart do not change with screen size):

Taken together, these two settings say:

Position my pivot exactly 56 pixels left and 259 pixels below the center of my parent.

So if your parent element is about 600 pixels tall, that nicely positions this item close to the bottom edge. But as the parent grows, the bottom edge gets further away from the center, leaving this object behind.

The solution: if you want something to align to the bottom of the window, you have to tell Unity that's what you want. It's not a mind-reader. 😉

Click the box that says "middle center" in your example to pop out the anchor presets picker, and choose one of the presets that aligns the object to the bottom of its parent:

Then adjust your positioning accordingly.

If these text elements are children of another UI element, not directly parented to the Canvas, then be sure that your parent is configured to stretch to fill the window - otherwise you could end up correctly aligning to the bottom edge of a parent, which is in turn centered on the screen, keeping your original problem at one level of remove.