Hello F.,
A possible workaround, in order to use dotted (or dashed) arrow on sequence diagram, without use of '-->' will be:
@startuml
Foo -[dotted]-> Bar
@enduml

[Click to see on the PlantUML online server]
Compare the same result with '-->' and with '-[dotted]->':

[Click...]
If that can help,
Regards,
Th.