I cannot figure out why {{lookupissues.size}} returns 0 for me.
I have my rule set to manual trigger to test it and then have the following setup.

I know that if I run the JQL query on my ticket (substituting {{issue}} with the actual issue key ,I get 4 results

the log then shows 0 for my {{lookupissues.size}} value:
I am owner of the project and the rule is run using my account. I must be missing something obvious but not sure what