Displaying PNG in large rectangle causes edges to be cut off

Hello community,

I am current having a problem where I am trying to put a PNG file over 3D Tiles to color them (Flood danger level with 3D display).
The files are both created via FME, but when displaying the PNG file with the exact lon/lat data inside a Cesium.Rectangle, it seems like the edges are cut off, and/or the image is being a little bit distorted, as it partly fits the 3D Tiles, but also partly gets shifted left, right, up or down.
Is there any particular reason for this? What causes the distortion and cutting off of the edges and is there any way to fix it?

The PNG files are currently split into 4 pieces, because the pixels were over 16000. Looking at the files themselves they fit together perfectly, but when you look at the connection in the Cesium viewer (screen attached), the transition is a bit cut off and thus the transition looks very incorrect. Also note how the left part is shifted to the right, and the right part is shifted to the left of the 3D Tiles beneath.

Any help would be much appreciated.