The first step is to start an answerhub bug report or BP thread. If you can narrow down the reproduction case it will help. ie, if you make a new blueprint that only has one or two variables and make new class BPs based on that, can you still reliably get it to lose data after restarting? If you can come up with a simpler repro case it will greatly increase the chance that somebody here will be able to reproduce it and fix it.
Usually it is more likely somebody will try to reproduce the bug themselves based on the steps you give to reproduce it, but if that turns out to be impossible, you could upload file to github or some other file sharing site in order to share them with us for debugging.
Include any and all relevant information including UE4 version etc.