WM tiled set output to Terragen 3

Am new to WM having bought it a few hours ago.
Not sure if this a bug or if I’ve missed some setting somewhere.
I built an 8x8 set of 4k heightfields and tried to export the lot to Terragen format.
The resulting WM-generated .tgd file is in a binary format yet .tgds saved from Terragen are ASCII (and therefore editable).
The tgd won’t open in Terragen 3.
Are there existing problems with WM exporting to Terragen or did I miss something?