I've booked the following revenue Economy itinerary last year:

26.04.2024 LH1661 + LH452
05.05.2024 LH459 + 06.05.2024 LH1660 (not really relevant for my question)

Due to a schedule change to LH452 a few months ago, I took the opportunity to rebook using the alternatives that have been offered in the app and ended up with:

26.04.2024 LH1661 + LH95 + LH456

Before rebooking, I was able to upgrade both outbound legs as well as inbound legs at fixed price. After rebooking, I'm only able to upgrade the inbound legs, which haven't been rebooked. I still see the option to bid for all flights, but as far as upgrading for a fixed price, this isn't possible.

I've asked 2 different LH employees through chat why this is and their answer is that all these flights are fully booked. Using EF I can see that this is clearly not the case. I've also asked if the ticket hasn't been correctly reissued after rebooking and they said that the ticket is fine.

Do you guys have any clue what could be going on? Should I call a specific call centre to get better trained CS representatives?