I used your source WAV file and test montage and I still can’t reproduce the problem. I watched the screencast but I do not know the language that you are using in Wavelab so I can’t comment on all your render settings.
Opening your montage and rendering a level 8 Flac file with my render settings attached in the screenshot produces a 48k Flac file. Also, doing your workflow of opening the 48k WAV in the edit mode, and creating a montage from the current file, and then rendering to level 8 Flac file also produces a 48k Flac file.
Here is a link to both of the rendered Flac files I made from the methods described above:
File A is direct from your montage, file B is making my own new montage from the source file in the edit mode.
