I’ve had some accidents with glyphosate. Once I planted about a half acre of sweet corn too soon after spraying a glyphosate burn down and pretty much none of the corn came up.
It helps if you don’t get a lot of rain immediately after spraying, so the glyphosate can have a good chance to bind with the soil (rendering it inactive).
It also helps if your soil is heavily mineralized. Glyphosate is a chelator and readily binds to certain minerals. If your soil is heavy in Ca, Fe, Mg, Mn, you’re going to be better off. If your soil had a lot of sand, that’s bad for rendering glyphosate immobile, and potentially bad for hibiscus.
All that said, your probably fine. I’ve sprayed glyphosate right over the tops of buried peach seed, and they come up fine.
If you’re really worried and it hasn’t rained yet, you can scrape the top 1/2" of topsoil from around your hibiscus plants. That would remove any chance of the glyphosate moving down and being absorbed by the roots. But like I said, I doubt your hibiscus will be harmed if they are fully dormant.