Trigger on Feeditem is not working as expected.
Triggers on Feeditem will only work when added to Feed Posts (Text post, Link post, File posts,...). Feed Events (Email message event, Create record event,..) however do not work. Feed events are always backed-up by another entity. To overcome the problem, you can write a Trigger on the backed-up entity like EmailMessage for Email message feed event, you can add the trigger to that entity directly instead.
Moderate Feed Items with Triggers
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