It's optional, although it's enabled by default in the LLVM and Chromium styles.
It is quite common to have an auto-generated configuration header, for instance; or a precompiled header; or optional headers that, when present, mutate the behaviour of other headers.
Every time you run a configure script for a C project there's a good chance you're interacting with code in this way.
It's not like clang-format was written by idiots who never used the language before. Maybe read up on what the tool actually does before you get all mad at them?
I think it's a harmful anti-feature that's worthy of criticism.
This helps ensure that your specific project level header files have all the necessary forward declares and includes to be a fully functioning and complete header.
In the end though you'll have to make a decision on how to sort the projects and which one is more or less specific than another. Kind of like deciding to just sort by pointer address if all else is equal.