In 4.26.2 version. It happened to me to have disconnected foliage from the landscape, the reason is that I renamed the LandscapeStreamingProxy object in the World Outliner. This seems to cause all foliage on it to actually disconnect, causing the same issue mentioned above: If you try to sculpt the foliage will not follow anymore the ground.
The only way I managed to fix it is to “select invalid foliage” (its a button in the toolbar when in Foliage mode), then move all of them up with the select tool, and then press “End” on your keyboard, which is forcing them to reconnect to the ground. This worked for me, but will be better if the bug was fixed somehow.