Safe is just not on after a while of use. Also there is no "battery manager > protected apps" in Honor 8 Lite menus
Maybe good to contact their (F-Secure) official Support Channels:
chat as example (when chat is available).
With two concerns:
-- Knowledgebase article is not applied to all devices / situations;
-- To receive valid workaround or confirmation about likely fixes;
Just as own steps: maybe possible to doublecheck if your device with equivalent of noted options (or ability to choose F-Secure SAFE as 'privileged' application against its termination).
Otherwise - good if F-Secure Team with abilities to add something (or it is provided by Android platform) against this kind of situations.
I'm really shocked to discover that this bug is still not fixed. This thread started in May 2018 and now it's March 2019.
Don't developers understand that this BUG makes the app ABSOLUTELY USELESS???!!!