Every use of mCustomData is not tagged with UPROPERTY() and thus makes the mCustomData Object able to be garbage collected if that is the only pointer to it. This crash is kind of hard to catch since garbage collection takes place every 60 seconds and sending a http request has a pretty fast turn around time. We have experienced this in our live builds.
This is the first post I was able to find that mentions it and it is from almost 4 years ago.