Drop a Button.
In Inspector use cog menu for Text property to select "Replace with Internationalized String".
In Hierarchy enter inline edit mode and type "%whatever" ==> BUG in Inspector one reads
Text % "%whatever"
Only one percent sign should be displayed but there is an extra one within the text field.
The good news is that the fxml file contains a correct syntax with only one % character.

For some reason if you do exactly the same inline edit but this time on Content it is doing fine.
Unclear if it is a Hierarchy issue or an Inspector one ...