qmake: how to debug the "No rule to make target" error?

[Violet Giraffe]

I'm getting the following error when running qmake on my project:

Error: dependent 'sub-C--DEV-cpp-db-tests----cpp-db-pro-qmake_all' does not exist.

Update: CONFIG+=ordered solves the problem, but I still wonder what it is. I don't like using ORDERED. I've triple checked all the dependencies (by all I mean both), there's no problem there.

This happens when running qmake with the default generator (Makefile) on Windows or Linux. At the same time, the project tree does load completely into Qt Creator, and generating the solution for Visual Studio works without issue.

I do not understand where the problem is coming from, the error message couldn't be more useless. I have created countless projects of this kind over the years, and I can't see any silly mistakes in this one. Yet, apparently there is such a mistake lurking somewhere.

Here's the root of the project in question: https://github.com/VioletGiraffe/cpp-db/blob/master/tests/tests.pro
It depend on two more public projects being deployed one level above this one (so that the three repos are side by side, 1 and 2). You can clone them and try for yourself.

How can I go about troubleshooting this error?
Usually, when there's a problem with a link to a sub-project in a SUBDIRS solution, the faulting project simply isn't loaded in Qt Creator and you can easily see what's missing. But not the case here.