UV coordinates are inaccurate Edit: Thank you @Gorgious, scaling the points to the 2D cursor worked. I hope blender updates Constrain to image bounds to work properly with UDIM tiles.

I'm trying to set the y-coordinate of a bunch of vertices to 0. (Some of my vertices are less than 0 and it causes issues in external software.) I select the vertices, and set all their y-coordinates to 0. I should not have to worry about floating point inaccuracy since 0 has a perfect representation. Yet entering 0 as the y-coordinate does not set them all to 0, most are a bit above or below.
If I try to set them one by one, the first one gets set to 0 properly, but then setting the adjacent one to 0 messes up the previous one again. What on earth is going on? Attached a picture of my UV layout and the vertices I'm trying to set.
Like most other fields in blender when you modify this value, you actually modify the mean (not median as sometimes stated in the tooltip...) value of all the vertices, not all their individual values.
You can use the scale transform tool to scale everything along 0. Use the 2D cursor as a pivot point to do it :