Question auto generated offmeshlinks are only working one way

Hello,
I am using offmeshlinks (auto generated and manual) successfully on my proto.
But I have a new test scene with some strangeness: some offmeshlinks, autogenerated are grey in the preview in the scene view. And so are invalid. Don't know what it means, cannot find anything about it in the doc. The links with this problem are ok one way (black or blue not sure) and invalid in the other way.
All this with Unity 2021.2.

Any idea ?