I repaired some holes in a wall that is to be wallpapered, and then
started applying "Shieldz" to provide the proper surface for the
wallpaper -- and now, an hour or so later, I find that the drywall
compound is lifting up and falling off at the nicely tapered edges. The
dry-wall compound has had a couple of days to dry.

What have I done wrong?

Perce

I couldn't say for sure, but perhaps the joint compound needed to be primed first before applying any other coating to it. Try contacting the manufacturer.