Bug in "Merge Channels" with large files

I’m seeing an odd but consistent error in merging 3 large single channel images. When I tell it to merge to an RGB, it rearranges large pieces of the image to be in different places. When I tell it to make a composite, it also rearranges the image, puts in random lines, and places small zoom-outs of 1 or 2 of the channels in the final image. It does this quite consistently (repeating it with the same files will always give the same result.) If I save the file and reopen it, it will look different but still wrong. What’s weird is the thumbnail of the saved image is correct, so clearly FIJI knows what the file should look like. Merging a small piece of the file works fine. Splitting after merging produces the correct images. It’s just the RGB merged or composites that are wrong.

I’m using ImageJ 1.51h. These images are 1.1 Gb each, but I have 128 Gb RAM, so that should not be a problem. If it’s relevant, the images were generated with Grid Stitching.

I’ve attached a screen shot of the 3 single color images and the buggy composite image.

Does anyone know how to fix this?