Empty contact and chat list on launch after phone reboot

Good day guys, There's this bug that we encountered that we can't seems to reproduce again. I just want to ask if you guys have an idea how this occurred or is this really an issue. It goes like this. We're using our messaging app then suddenly the battery died. We charged the phone till it opens up. Launch the app again. Chat list and Contact list were empty. We need to relaunch the app again to populate the list. The device we used is an iPhone X. Sorry if my English is bad. Thanks guys!