As a matter of fact “spawn sound attached” (put right before the timeline) also goes out of sync. It goes out of sync so hard I can listen to it live shifting away while my previous method kept the out of sync steady 
However, I did some brainstorming and went through all my existing blueprints in my head to “browse” in which ones I also have moving meshes with looping sounds that rely on “if mesh is there, stop sound, if mesh moves again, continue sound”.
Surprisingly and gladly I did setup such a blueprint in the past where the sound never went out of sync. I don’t even remember exactly why I set up this BP differently but I did, so I carried that style over into the current BP that makes sound sync difficult.
And voilá - here we go, sound now is and stays in sync 
My solution is this: Split timeline in half (so only one-direction-movement), split sound in half (so to say) and code logic for a timeline that is not looping with manual play and play reverse execution orders, like so:
-happy-