Seasonal closures affect not only seasonal road closures (mountain passes and the like) but also closures due to roadworks. This is why the route takes you through the closure even during planning, even though Here is actually aware of it.

[Peter Zehentreiter]

I’ve noticed the following... when I plan a route and enable seasonal closures in the settings, the seasonal closures of the passes are taken into account, but the closures due to temporary roadworks are also ignored, and the route is strictly directed through the closure (even though the closure is known to Here)

That’s why the route already takes you through the closure during planning, even though it’s actually known to Here.

However, if I deactivate seasonal closures in the web planner, the roadworks are taken into account during planning and a detour is created at that stage.

Is this a bug or is it intended behaviour?

Ideally, roadworks closures should be treated separately from seasonal closures.

When I open and start the route in the app, the app recognises that there is a closure, alerts me to this with a warning and also creates a diversion.

Why does the behaviour differ between the website and the app?