I know that dependencies between projects have to be processed sequentially, in fact I'm talking about parallel build within the single project. It is working if I use just "-j" without specifying the number of jobs, so I would expect that it worked similarly even with "-j".
I already reached the Android NDK forum and they think that the issue could be related to QMake (obviously), but the most reasonable argument right now is that, as you already said, QMake just prepares the Makefiles, any issue with the parallel building should be caused by Make itself.
I'll try to find an example project to build on my machine in order to reproduce the issue, so that I can share with you and with the NDK developers.
Thanks a lot for the moment!