CLI tab completion cannot be used in the middle of the command. Whole command is taken as a context, ignoring the current position of prompt. Auto-completed string is inserted to the current position of the prompt (no matter it can be placed in the middle of the command).

[] is used to show the current position of the prompt
actual
tab completion in the middle doesn't work

/subsystem=loggin[]<TAB>/root-logger=ROOT
/subsystem=loggin[]/root-logger=ROOT

tab completion with prompt in the middle

/subsystem=logging/[]<TAB>root-logger=R
/subsystem=logging/ROOT[]root-logger=R

expected
tab completion in the middle works

/subsystem=loggin[]<TAB>/root-logger=ROOT
/subsystem=logging[]/root-logger=ROOT

tab completion with prompt in the middle doesn't add anything unless it's a valid part of the command

/subsystem=logging/[]<TAB>root-logger=R
/subsystem=logging/[]root-logger=R