Currently, headers install is broken in some stable builds. Packages are not there for a reason - I don't want to update stable repository after enough testings. Since testing reports show that most of the things work ... with one known exception which needs to be fixed (MiQi) ... I'll run the major update as soon as I am back in the office. ATM I have limited time to deal with possible/expected problems.
Workaround to get headers is switching to the nightly build (preferable with armbian-config), which is mostly already at the "next stable" level. DEV kernels, which we are talking about here, are ofc yet another exception. They are still under development and nightly versions might be broken ... or not.
If you are already switched to the nightly and it is not possible to install headers, then this an unknown bug for me.