@PrincessDonna: If the issue has persisted following a factory data reset, then please try uninstalling any 3rd party apps you have added since the reset, to explore the possibility that the problem is stemming from an external source. I also recommend heading to Settings > Battery and device care > Optimise now, followed by Memory > Clean now. You may also wish to tap on Device protection > Scan phone, to check for any malware or suspicious activity. Furthermore, please head to Settings > Apps > Select an affected app (e.g. WhatsApp) > Force stop > Storage > Clear cache. Let me know how you get on.