index threshold rule does not work as expected with a threshold below a value

[daverick]

Kibana version:
7.13.2
Elasticsearch version:
7.13.2

*Original install method (e.g. download page, yum, from source, etc.):
ECE 2.7.1

Describe the bug:
A index threshold rule with a condition below a value is not active when it should be.
A similar Elasticsearch query rule with a condition below a value is working as expected.

Steps to reproduce:

1. Create a index threshold type rule on a index with a @timestampfield but no recent data in it. Use a count() over all document and a condition is below 1 over the last 5 minutes.
2. Save the rule.
3. Go the the Rule and connectors and look at the created rule. it status is OK

Expected behavior:
the status should be Active

Screenshots (if relevant):
rule configuration where some test data can be seen. no alert was triggered.

Errors in browser console (if relevant):
no error in browser or server logs

Any additional context:
The purpose of this rule is detecting when no document are ingested in some index that should continuously received data.
condition is above works as expected.
condition is between as the same problem.

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)

[mikecote]

Adding to To-Do to research if it's a bug or a no-data scenario. No-data scenario may require more work and re-prioritize.