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:
# create a message with a template with parameters which needs to be substed (I've copy/pasted it): `{{subst:Avertissement suppression page|Testdetest}}`
# 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
# `{{subst:Avertissement suppression page|Testdetest}}` is displayed on the text area instead of the template itself
# save: template is substed and displayed
Tested with the template `{{subst:Avertissement suppression page|Testdetest}}` [[ https://fr.wikipedia.org/wiki/Sujet:Srr6xtpqbicwxlbb | on frwp ]].