I’m also having this problem and it looks like it’s an engine bug. I have a Niagara system with a single looping particle that only begins working in game after I open up the effect.
After I open up the effect it works the entire time the editor is open, but when I close the editor and open it back up again, the effect stops working again. The effect does not work in packaged executables either.
If I make a package AFTER opening the effect to get it working again, then the built package fails to start when I run it and I get a LowLevelFatalError from line 1161 in AsyncLoading2.cpp stating “Bad export index”.