I finally found the reason.
That was a long-lasting issue I had, and I can see more people are having this.
The language string has been changed. But because the variable name is the same, it does not come out as a missing translation in Pontoon.
Old form:
listen-empty-state = We’ve run out of clips to validate in this language…
New form:
listen-empty-state = We may have run out of clips - refresh the page, or try again later.
You just need to go to Pontoon, search for “listen-empty-state” and re-translate it accordingly…