After a discussion on T116417#1761384 and a test, it appears that templates which should be substed are not displayed as they should be until you post the message.

What happen:

1. create a message with a template with parameters which needs to be substed (I've copy/pasted it): {{subst:Avertissement suppression page|Testdetest}}
2. a message displays (very quickly "template is converted to wikitext" or something like that) - actually when it works: on three trials, I only had two conversions
3. {{subst:Avertissement suppression page|Testdetest}} is displayed on the text area instead of the template itself
4. save: template is substed and displayed

Tested with the template {{subst:Avertissement suppression page|Testdetest}} on frwp.