I tried your solution too, it came with this response:
pi@pi-hole-ellen:~ $ sudo apt-get update --allow-releaseinfo-change
Geraakt:1 http://archive.raspberrypi.org/debian buster InRelease
Ophalen:2 http://raspbian.raspberrypi.org/raspbian buster InRelease [15,0 kB]
Ophalen:3 http://raspbian.raspberrypi.org/raspbian buster/main armhf Packages [13,0 MB]
13,0 MB opgehaald in 25s (522 kB/s)
Pakketlijsten worden ingelezen... Klaar
N: Bij pakketbron 'http://raspbian.raspberrypi.org/raspbian buster InRelease' is de 'Suite'-waarde gewijzigd van 'stable' naar 'oldstable'
pi@pi-hole-ellen:~ $ pihole -v
Pi-hole version is v5.4 (Latest: v5.4)
AdminLTE version is v5.6 (Latest: v5.6)
FTL version is v5.8.1 (Latest: v5.9)
pi@pi-hole-ellen:~ $
So, the FTL is still 5.8.1..
Now I did a pihole -up....
After a while and a lot of responses, it said:
Update Complete!
Current Pi-hole version is v5.4
Current AdminLTE version is v5.6
Current FTL version is v5.9
pi@pi-hole-ellen:~ $
So thanks for helping the nerd in me out!