This is normal Lightroom behavior, not a bug. In Library, some actions behave differently in Grid vs. Loupe, and batch keywording from Loupe doesn’t work the same way by default.

Workarounds:

1. Switch briefly to Grid view for multi-photo keywording. This is the simplest workflow and is how many users handle batch operations.
2. In Loupe view, use Auto Sync:
   • Shift-select the photos in the Filmstrip
   • Turn on the switch next to the Sync button so it becomes Auto Sync
   • Add keywords in the Keywording panel Lightroom will then apply the change to all selected photos, not just the active one.

One caveat: if only some of the selected photos already have a keyword, Lightroom may show an asterisk next to that keyword, indicating it’s applied to only part of the selection. In that case, adding the same keyword may appear to have no effect until the selection is made consistent.

So yes: by design, and Auto Sync is the main way around it in Loupe view.