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?

[Jörgen]

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

That's what I would expect, too.

[Marinus van Deudekom]

Nice find
I guess that one of the devs wel i comment on that

[ginogino]

I'd say the accuracy of seasonal closures and roadworks is correct in 3 out of 4 cases. On a road I used to travel a lot and which will now be closed for several months due to a landslide, myrouteapp web won't let me pass through it. On another road that passes through a mountain pass, it lets me pass through if I disable the seasonal closure, but that road isn't closed seasonally.
I once happened to be driving a route I usually take and while navigating, myrouteapp next kept telling me to turn back. But since I'd driven that stretch of road a few days earlier and know it like the back of my hand, I thought myrouteapp next had crashed. Instead, I discovered that that day, and only that day, the road was closed.