How can a tabelt forget the SSID of a wifi network

We have a product that combines one Galaxy Tab 3 7"or Galaxy Tab 4 7" tablet, both wifi-only, with a Mikrotik RB951G-2hnd router. The Mikrotik has a hidden wifi network so we add it manually to the tablet. Most of the time it works great but for some reason, sometimes the tablet forgets the SSID of the hidden network. According to our costumers, this happens when there's a power cut and the tablet looses its charge. After the power comes back, the tablet won't connect to our mikrotik device. When checking the WiFi connections on the device configuration the network won't even appear on the list. This happens only sometimes in some tablets. How can the device forget a manually entered SSID?