one thing you could try is to run sslvpnd debug on the FFortiGate and then wait for the issue to occur.
diag debug console timestamp enable
diag debug app sslvpn -1
diag debug duration 480
diag debug enable
diag sniffer packet any 'host <your_android_ip>'
Then see what happens. Maybe your android does a network change or has the option enabled to run connectivity via both Wi-Fi and cell network. If the wifi is bad, the mobile could be switching to the cell network IP/adjust the route, no idea how it does that. The SSLVPN would then be disconnected.
Depends on the phone model on where you can disable/check the status of the setting. Some have it hidden in the developer options.