With Picard 2.3 the behavior of the Delete key has changed.

Based on what is selected in the main window, the release or just track(s)/file(s) will be removed, also if the cursor is in the metadata view, i.e. a line/tag in the metadata window has been activated/changed.

In 2.2.x the behavior was different: If a line in the detail window is selected it's possible to delete this tag with the ␡ key.

I guess it's a bug, since I found no change log, I found no option to set the behavior and I didn't change my settings and kept it while install/upgrade.