Thanks for the bug report - I’ve been able to reproduce it with another Quixel image. It looks like this is a bug in the FreeImage library nvtt_export uses to load EXR files: when the input has only 1 or 2 channels, it tries to load the “Y” channel of the EXR file. However, the single channel in Quixel displacement maps is named “A”; since OpenEXR can’t find a channel named “Y”, it fills the channel with a default value (in this case, black).
I’ll be working on a fix for this (a workaround is to open the .exr file in a hex editor and change byte 0x1C from an “A” to a “Y”, but that’s not a very easy workaround).
Quick update - we released Texture Tools Exporter version 2023.1.0 today, which now includes a version of FreeImage with this fix! When loading an OpenEXR file with a single channel, it will now load that channel, regardless of its name.