The standard undertray fittings are absolutely hopeless when the lugs for the Dzus fasteners corrode they don't hold, those LandRover ones are no better as they don't fit on some of the subframe fittings
What you really need to make the undertray secure are some of these instead of the standard clips:
https://www.ebay.co.uk/itm/M4-M5-M6-...S5sB70YEcXdGKA
M6 should do it combined with some M6 bolts and some suitable repair washers
It shouldn't come adrift then
My Volvo has them as standard for it's undertray and they do a far better job, they've never come adrift
Russ