Setting up a scatterer on the timeline with a loop region should work. There are a few things you can do to track it down:
1.) Play the event in FMOD Studio and make sure it still plays and produces sound.
2.) Build banks then open the FMOD Studio Sandbox and drop the event in there. The Sandbox simulates a game more closely, including the stopping behaviour. If you play the event in the sandbox, see that it stays playing, and produces sound. If it stops there, make sure you have your loop region correctly set up on the timeline.
3.) Run UE4 PIE and connect via FMOD Studio, then record a profile session. After the session finishes, you can look at the lifetime view and see that the event stays alive as you expect. You can also see the metering levels to make sure it is still producing sound, and the channel count to verify it is still playing channels (voices).