Some time ago I submitted a bug report of the subject. Unfortunately it didn't get enough attention.
Initial thin archive support was added to llvm-ar by Rafael Espíndola, who left the project and probably doesn't want to be contacted concerning this problem.
I'm writing to the mailing list in order to attract attention to the problem, ask if anyone can reproduce my findings and check if they are even relevant.
The link to the original bug report: https://bugs.llvm.org/show_bug.cgi?id=37530