This was a known problem in a previous version, and was supposedly fixed (I confirmed that it was fixed), but I tried it recently too and it's broken again. I was feeling disheartened at the time and did not bother to file a bug report, but if you want to:
New topic for Photoshop Family
If you really need to discard 1:1 previews of selected photos, you can do so using PreviewExporter's "Discard Lib Previews" function (free plugin I wrote), but note: it discards standard previews too, but you can rebuild them. You probably already know this, but if you want to discard all 1:1 previews, trash the entire preview folder (.lrdata) and rebuild standard previews. (I realize you'd rather not have to take the time to rebuild standard previews, but that's the best I can offer..).
Cheers,
Rob