have you tried opening the page in the debug mode in Matomo Tag Manager?
When you klick on the submit button with right click and you open the data layer for the specific click event you see which data matomo pulls from that button click and therefore which data you can reach with a trigger. It can happen that matomo can’t extract the click text from the button but something else.
Thanks a lot for your answer.
I’ve tried with Firefox and finally it works… Really strange. Maybe, I 've done so much test with Chrome than my cache was creating troubles…