As @vdbhb59 already mentioned, there is a workaround to push the files in the right places by using adb push. The whole procedure is described here: https://forum.f-droid.org/t/privileged-extension-ota-workaround-for-los-17-1/11058
I have used this workaround successfully on several phones with official LineageOS 17.1 and 18.1.
Since I am not a developer, I don’t have a clue, why there is no fix to make the ota.zip work like in earlier versions of LineageOS.