I have also observed this behavior, and just did some tests to confirm loss of data, and yes-- unfortunately that is happening. During import, I did various adjustments on images just imported, and did an undo on the adjustments. About half the time, along with the proper undo, LR actually did an undo on the metadata that was being applied during import . Multiple consecutive images were affected, and multiple metadata fields in those images were blank when the import completed! So if you have run into this scenario, check your metadata since some is likely missing.
"Daft" seems applicable, along with bug. This is one more instance of serious issues with undo that have been reported in the forums.