The OS takes up space as well. And, if you are transcoding, those would get written to disk. /var/lib/jellyfin/transcodes, by default. If transcodes are the culprit, Dashboard > Playback has a setting to change where transcodes are written.
Or increase the size of the root volume again.
Jellyfin 10.9.11
Ubuntu 24.04 LTS (bare metal)
Intel i3 12100 on Asus Prime H610M-E D4 mATX
32GB DDR4-3600
Intel Arc A380
OS drive - SK Hynix P41 1TB
Storage
WD Green 3TB (Samba shares)
WD Red 3TB CMR (WIP Media, Test libraries)
3x WD Red Pro 6TB CMR in RAIDZ1 (JF Library)
Fractal Meshify 2
Corsair CX430