This LX thing is a known behaviour. Since there is no F service LHR-ZRH it will use C as the major cabin. Just overwrite the used cabin in the upper right.
You can do this on the fly - no need to reload the page, as the links get recreated when updating the cabin.
Originally Posted by Wiki of this thread
Cabin detection:
You should set the cabin on the upper right to the appropriate main cabin you are looking for. Sometimes you will have economy/business class segments in mainly business/first class itineraries. These mixed itineraries make it hard to autodetect the main cabin - so you can set it manually.
Once it is set to First, the link will bring up your desired itinerary.
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