On file [Engine\Plugins\FX\Niagara\Source\NiagaraEditor\Public\ViewModels\Stack\NiagaraStackGraphUtilities.h], function FNiagaraStackGraphUtilities::FindScriptModulesInStack() receives a parameter "OutFunctionCalls" which is intended to be a function output, but it is being passed by value instead of by reference.
This function was created in 2017 already with this bug, which still exists in mainline latest. The only callers UNiagaraScriptSource::AddModuleIfMissing() and UNiagaraStackScriptItemGroup::RefreshIssues() never needed to use the output, relying only on the return value instead. But licensees have run into the bug and reported it.
This bug can be verified by inspecting the related code.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-312484 in the post.