Hello T.,
A first workaround is to add some dummy spaces (~ 2 or 3 per days) before the task's labels, as:
@startuml
[Task1] lasts 1 days
then [ Task2_long_long_long] as [T2] lasts 2 days
-- Phase Two --
then [ Task3] as [T3] lasts 2 days
[ Task4] as [T4] lasts 1 day
[ Task5] as [T5] lasts 2 days
[T2] -> [T4]
[T2] -> [T5]
[ Task6_long_long_long] as [T6] lasts 4 days
[T3] -> [T6]
[T5] -> [T6]
@enduml
[Click to see on PlantUML server]
If that can help,
Awaiting an automatic positionning,
Regards,
Th.