I'm having trouble creating a B/W preset, using the latest Lightroom for mobile (iPad Pro '11).
Firstly I edit my image using Gray mix to bring down luminosity of blues, and push up oranges and greens for skin & foliage. Then, when I generate my preset I make sure that 'Black and white mix' is ticked (under 'Color').
The newly generated preset will then apply all my setting EXCEPT my Gray mix values.
Looks like a bug. Just tried it and it on mobile Lightroom and it doesn't generate a preset with the mix values embedded. You should report the bug at https://feedback.photoshop.com. It also doesn't work correctly in Lightroom Desktop.