You can look at the: Process SLAs business rule to help you glean into the code that's being ran.
In short you might be running into an issue where due to the async processing; the Journal Field type may not maintain the changes flag?
You could test this by picking some other field (like state) and using the "Changes" operator to see if it works. If it does, that would help confirm the hypothesis above. If this ends up being the case.. You could work around that with an alternative solution by having a "comments counter" field; that you increment + 1 everytime with an onBefore business rule ever time the comments changes. You can then use this field in your SLA Condition.
