There is a side-effect to using variables in logstrings:
plantuml -DSUBMODULE mymodel.pu
!log "SUBMODULE"
!log "SUBMODULE2"
The output is:
[LOG] ""
[LOG] "SUBMODULE2"
Same unexpected output would appear in case of this kind of logstrings:
"Variable SUBMODULE has a value %get_variable_value(SUBMODULE)"