Hello Gustavo!
I believe you've run into the issue reported here: index threshold rule does not work as expected with a threshold below a value · Issue #103922 · elastic/kibana · GitHub
We've fixed this in the master branch with this PR: [Alerting] Allow rule to execute if the value is 0 and that mets the condition by chrisronline · Pull Request #10
According to the labels in the PR, this fix will be available in Kibana 7.14.1 and 7.15.0, when released.