DBDSDC returns non-orthogonal U and V for a specific matrix

[sergey-v-kuznetsov]

I'm attaching a Fortran code which demonstrates the problem. The code reads two diagonal matrix from a file in the binary format and call DBDSDC:
CALL DBDSDC( 'U', 'I', N, S, E, U, M, VT,
$ M, DUM, IDUM, WORK(INIAD), IWORK,
$ INFO )
but DBDSDC returns non-orthogonal U and V. It should be noted that DBDSDC returns orthogonal matrices if we slightly change any diagonal entry.

Here are steps to reproduce:

bash-4.2$ ifort -V
Intel(R) Fortran Intel(R) 64 Compiler for applications running on Intel(R) 64, Version 18.0 Build 20180611
Copyright (C) 1985-2018 Intel Corporation. All rights reserved.

bash-4.2$ ifort reproducer.f ../lapack-3.7.1/liblapack.a ../lapack-3.7.1/librefblas.a
bash-4.2$ ./a.out
Check orthogonality of U
Orthogonality test failed
Max element in U^T * U - I 0.999872111722121
Row and column numbers of max entries 619 620
Check orthogonality of VT
Orthogonality test failed
Max element in U^T * U - I 0.999872111722121
Row and column numbers of max entries 619 620

If we slightly change the last diagonal entry by inserting
s(n) =s(n)+1.D-15
the attached test passes:
bash-4.2$ ./a.out
Check orthogonality of U
Orthogonality test passed
Check orthogonality of VT
Orthogonality test passed
Repoducer.zip