Reg 5.12.0a -> 5.12.0b1 qmake fails on SUBDIRS projects with -tp vc

This problem appeared in Beta 1, Alpha was working fine. Running

qmake -tp vc -r

to generate a 64-bit Visual Studio project file on a SUBDIRS Qt project results in the following error output:

ASSERT: "&other != this" in file c:\users\qt\work\qt\qtbase\include\qtcore\../../src/corelib/tools/qstring.h, line 958

As a result, the .vcxproj files for the sub-projects seem to be generated, but the .sln file is 0 bytes in size.

Related observations:

1. The problem only occurs with the Visual Studio project generation (qmake -tp vc -r), not Makefile (qmake -r).
2. It only occurs with the 64-bit Qt version (msvc2015_64), not 32-bit (msvc2015).
3. It seems to occur on every single SUBDIRS project. Attached is the empty project structure that reproduces the error for me.

And here's the exact command line I use to invoke qmake that results in the error:

call "%VS140COMNTOOLS%..\..\VC\vcvarsall.bat" amd64 && C:\Qt\5.12.0\msvc2015_64\bin\qmake.exe -tp vc -r