I have confirmed that my (and several other) extensions are not built with the build system. I have set up my own build machine, and several extensions are not cloned, and my build log has entries for configuration (of all extensions in the extension index), but nothing for building/testing/etc. (of a few extensions). There is nothing in the build log explaining why they aren’t built (that I could find via search).
We experienced the same. Do the extensions that are not built depend on other extensions? Do the dependencies have failing tests? Jc said he fixed the problem of extensions not being built if extensions that it depends on have test failures; but there may be still similar problems.