Bug - Return the Favor is not copying Arcane Bombardment's triggered ability
Return the Favor should be triggering Arcane Bombardment's ability again. It works when casting Return the Favor from your hand, but not when being triggered by Arcane Bombardment. When it should show the copied trigger of Arcane Bombardment, the game seems to just skip that step and move on. This is functionality that recently used to work: video example attached.

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RedMag3#75915 commented
This is intended behaviour, to get what you're trying to do to work you need a spell doubler like Chandra, cast Return the Favor on something with Chandra and Bombardment out, the copy of Return the Favor can copy the Bombardment trigger.
You will now have two Bombardment triggers, resolve one and use a Return the Favor under Bombardment and target the ORIGINAL Bombardment trigger, the second one will dissappear before this Return the Favor resolves.
So if you copy the original Bombardment trigger again it will make a second Bombardment trigger, as long as you have a treasure producing spell resolve before Return or some cost Reduction like Haughty Djinn you will go infinite.
Hope that helps.
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Rezzahan#77802 commented
No, it shouldn't trigger again. Because:
"Whenever you cast your FIRST instant or sorcery spell each turn ..."
Also, targeting the already resolving trigger with the copy mode will do nothing, since the trigger will be gone (resolved and thus removed from the stack) by the time Return the Favor would resolve.