Make sure the menu item Metadata > Show Metadata For Target Photo Only is unchecked.
johnrellis - thanks for the quick response!
That solved my problem on the MacBook running Lightroom 2015.3 - that setting was checked!
However, on the iMac running Lightroom 2015.6 it was't checked. I tried toggling the setting on and off but that didn't fix it. If I still select multiple photos and edit a Metadata field, the text I typed just disappears after I hit enter and doesn't apply to any of them. Yet if I pick one image and do the same, it works.
Any other ideas? Could there be another setting I'm missing?
Great. I was just about to suggest that. LR frequently soils its preferences and resetting can fix a surprising number of things when the user interface goes wonky.