There aren't any problems with IMAQ Multi-threshold and Float image types. I just tried it out, and it's pretty straight forward (I'm using Vision 6 for LabVIEW). Make sure the image type for IMAQ Create is set to Float.
I'm seeing the exact same problem. The only work around I was able to find was to convert the images to 8-bit or 16-bit images and then run the threshold. After that you can convert them back to floats. I'll inform R&D right now about the bug you found.