I could only find one case of the source buffer being AddUnititialized in FMixerSourceManager::ComputeSourceBuffersForIdRange (bare in mind again I am running on 4.16.3).
Changing it to AddZeroed didnt solve the issue unfortunately. I saw two other AddUnititialized in the audio mixer folder source code and changed them too just in case but still didn’t fix our issue.
I checked the unreal tournament and unreal engine versions of the AudioMixerSourceManager.cpp to see if I could work out what had been done but they still had AddUnitialized.