From the screenshot it looks like the session itself initiated on an accepted rule but no return traffic (0 bytes received) was ever seen.
this could be due to the server not responding or an asymmetric route sending the return packets over a different path.
The traffic is allowed as the syn packet hit a policy that allowed it to go out based on it's source/destination zones and port, but the session never completed appid so app based security policy could not be applied
if there was return traffic but we failed to properly identify the application the application would be set to "unknown-tcp" rather than incomplete.
For unknown-tcp we can apply policy based on application (so if you only have a web-browsing allow, unknown-tcp would be blocked as it is not an allowed application in your security policy)
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