FIXED! - In the Export Audio Mixdown window, make sure that ‘Insert iXML Chunk’ is NOT ticked.
The audio mixdown works great in 10.0.50. A 32 bit float 24kHz L/R wave file is created perfectly and can be opened fine in Wavelab 9.5. With Cubase 10.5, though, the mixdown file fails to open in Wavelab. Just nothing happens. When the file is loaded into Wavelab for batch processing, pressing the Start button pops up a ‘This file cannot be read’ message. Reverting to 10.0.50 with the same project solves the problem.