Tyre uses the maps from Google Maps.
This has advantages: The map is always up-to-date.
It also has a downside: if roads are temporarily blocked, you can’t use these for planning a route.
At this time this can be the case with mountain passes.

While you can set waypoints, the route won’t be shown on the map.
Annoying, but not a big problem, as your navigation system will recalculate the route based on the route points that you enter.
In other words: the route line that you see in Tyre is a handy tool, but it’s not necessary for entering the route correctly into your navigation system.

You can also place a route point each just before the mountain pass and just after the mountain pass.

Then select the route point before the mountain pass and click on 'skip'.

Tyre will then place a straight line to the next route point after the mountain pass.

In certain situations Tyre will automatically place a straight line.