In the past, with older Samsung devices, whenever I have enabled Secure Startup to fully encrypt a device, I have gotten an instruction to charge it to at least 80% and leave it plugged in, as it would reboot and take about an hour for this procedure to finalize (maybe not word for word, but iirc that was pretty much what it would say).
I've a used stock ROM Galaxy S9 here and I enabled Secure Startup, and it simply said that it is enabled... No restart, not an ~hour long activation process... Nothing, it just says it is enabled. When I restart the phone, it does ask for my password, but I suspect it just might be a UI/visual thing and that it does not really encrypt the device.
I checked if the Knox fuse was tripped and it wasn't. Also, because I had the phone heavily debloated, I suspected maybe I have uninstalled a crucial Secure Startup related system app, so I hard reset it and tried again with no default apps being uninstalled. But still the same thing. It says it is enabled, but did not say anything about charging the phone to 80% and that it would take about an hour, or prompt me to reboot, etc.
Again, this is a Galaxy S9 and I'm definitely sure it uses Full Disk Encryption (FDE) as its encryption method (I googled this). The reason why that is relevant, is because if it was File Based Encryption (FBE), I'd suspect it would be normal to enable Secure Startup instantly—as it seems to be doing it now—and not have to reboot and take about an hour to encrypt the entire phone.
What am I missing here?