Hello L.,
Just add ':' and ';' around 'i+=1'.
@startuml
start
if (i % 3 == 0 && i % 5 ==0) then (true)
:call subroutine;
note right
subroutine will be explained as a footnote
end note
endif
stop
start
:the subroutine;
if (%i == 3) then (true)
:i+=1;
endif
stop
@enduml
Then we can observe the expected result:
[See on PlantUML online server]
If that can help,
Regards,
Th.