A large header seems to cause a legend to be placed on top of the title. Check this example:
@startuml
title
Some Title
end title
right header
<size:16>Generated %date("yyyy.MM.dd 'at' HH:mm")</size>
end header
legend top
Some legend.
end legend
A -> B
@enduml
I've verified this with 1.2019.06.