Try resetting the preferences.
https://helpx.adobe.com/lightroom-classic/help/setting-preferences-lightroom.html
"It will apply the tag to only the image I am sitting on loaded in the browser. "
It sounds like you are in Loupe view (menu command View > Loupe, with one photo showing in the center panel), rather than in Grid view (View > Grid, multiple thumbnails showing in the center panel).
When you're in Loupe view with multiple photos selected in the filmstrip along the bottom, by default any keywords you apply to the photo will be applied to that photo only.
If you want to apply the keyword to multiple selected photos at once, you've got three options:
1. Go to Grid view.
2. Select the menu item Metadata > Enable Auto Sync (or click the Sync toggle in the lower-right corner).
3. In the Keyword List, right-click the desired keyword and do Add This Keyword To Selected Photos. (Note that checking the box to the left of the keyword only adds it to the most-selected photo.)
LR's handling of multiple selected photos is confusing and wildly inconsistent, such that not even experts can remember all the rules:
https://community.adobe.com/t5/lightroom-classic-ideas/p-consistent-behavior-when-multiple-photos-selected-in-loupe/idi-p/12360578
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