Look at the project in bug1 and see the difference for rules in Makefile that are generated for resource files aaa.qrc (not in OUT_PWD) and res.qrc (in OUT_PWD). There is no denendency generated for 1.txt in the rule for qrc_res.cpp and it does get generated for qrc_aaa.cpp.
res.qrc and aaa.qrc are indetical.

bug2 differs from bug1 only in that I've added 1.txt to PWD.
The dependecy for it gets generated! It's clearly a bug.

Notice - when .qrc is compiled files are found as they should, releative to resource file. The bug affects only dependency generation and not resource compiling. So I'm not sure who is in falt, qmake or rcc.