What you are reporting is not a bug.
The task is not to reach Paragon 400 or earn 400 Paragon points.
The task is to spend 400 Paragon points.
If you select all of the standard Paragon nodes in the Paragon trees you will, indeed, only have spent 360 Paragon points. However, there are three Legendary Paragon nodes that cost 80 points each to activate.
So, de-allocate 40 points worth of nodes in one of the standard trees (taking you down to 320 points allocated)
Activate one of the legendary nodes (taking you from 320 to 400 points allocated)…
That means you will have allocated 400 Paragon points, completing the Season Journey task.
