Now it's end of May, and this problem STILL has not been resolved by Microsoft, as my system failed today for the third time to install 24H2 and yet to retain my licence.
My Win11 was pre-installed when I bought the computer, and it has an official digital licence, hence all suggested attempts to retrieve a product key are bound to fail. All provided troubleshooting routines have failed. Microsoft was thoroughly unable to indicate any way how to get a digital licence back they have themselves killed. The only solution I can offer is this:
(1) Return to the previous build (in my case: 23H2). This proved to be the ONLY available way to get my licence back.
(2) Never try to install 24H2 again.
(3) Frack Microsoft.