Dependency/branch question

Project A with branch “special”, and set up with default branch “refs/heads/special” and branch filter “+:<default>”.

I only ever want to build Project A in the special branch.

Projects B, C, D, E, F each with dependency on Project A. Default branch and branch filter unchanged ("+:* for both default branch and branch filter). These projects only have a master branch and it should only ever build master.

I then created Project Z which is a composite build depending on B, C, D, E and F.

Z is either triggered manually, or on a schedule.

It seems that when Z starts, B, C, D, E and F sometimes want to build with explicitly the “master” branch which cannot then work with the dependency A, which can only build “special”. So the build fails.

On other occasions, it works; it appears that B, C, D, E and F build using the “default” branch, which is still “master” on B, C, D, E and F but, correctly, “special” on A.

Why is this and what can I do about it?