Hello,
Please see the following sequence and notice that incoming/outgoing arrow heads are inverted only if not connecting entities.

@startuml
participant Bob
participant Alice
Alice \\-: "\\\\-" Should be down
Alice //-: "//-" Should be up
\\- Alice: "\\\\-" Should be down
//- Alice: "//-" Should be up
Bob -// Alice: "-//" Is correct down
Bob -\\ Alice: "-\\\\" Is correct up
Bob \\- Alice: "\\\\-" Is correct down
Bob //- Alice: "//-" Is correct up
@enduml
Thanks!