libbrotli wasn't listed as a dependency for the AppArmor profile of the transmission-daemon binary.
As a result, transmission wouldn't run and would fail, logging this audit message to dmesg:
audit[11595]: AVC apparmor=DENIED operation=open profile=/nix/store/08i1rmakmnpwyxpvp0sfc5hcm106am7w-transmission-3.00/bin/transmission-daemon name=/proc/11595/environ pid=11595 comm=transmission-da requested_mask=r denied_mask=r fsuid=70 ouid=70
1bdda029cd