It seems zypper source-install claims it has installed sources without
notifying it actually was not able to, if either /usr/src/packages/ or
/usr/src/ directory is missing. However, if /usr/src/packages/ exists then it's
able to create the missing directories like SOURCES/SPECS.

In the copy-paste below, the source-install succeeds on the third attempt.

--- cut ---

timo@linux-sde2:~> sudo rm -rf /usr/src/
timo@linux-sde2:~> sudo zypper si workrave
Loading repository data...
Reading installed packages...
Resolving package dependencies...

The following source package is going to be installed:
  workrave

1 source package to install.
Overall download size: 6.2 MiB. Already cached: 0 B. After the operation,
additional 6.2 MiB will be used.
Continue? [y/n/v/...? shows all options] (y): 
Retrieving srcpackage workrave-1.10.44-3.2.noarch                              
                                               (1/1)
Retrieving: workrave-1.10.44-3.2.src.rpm
.........................................................................[done
(4.1 MiB/s)]

Checking for file conflicts:
.................................................................................................[done]
(1/1) Installing: workrave-1.10.44-3.2.src
...................................................................................[done]
timo@linux-sde2:~> ls -l /usr/src/
ls: cannot access '/usr/src/': No such file or directory
timo@linux-sde2:~> sudo mkdir /usr/src/
timo@linux-sde2:~> sudo zypper si workrave
Loading repository data...
Reading installed packages...
Resolving package dependencies...

The following source package is going to be installed:
  workrave

1 source package to install.
Overall download size: 6.2 MiB. Already cached: 0 B. After the operation,
additional 6.2 MiB will be used.
Continue? [y/n/v/...? shows all options] (y): 
Retrieving srcpackage workrave-1.10.44-3.2.noarch                              
                                               (1/1)
Retrieving: workrave-1.10.44-3.2.src.rpm
.........................................................................[done
(4.2 MiB/s)]

Checking for file conflicts:
.................................................................................................[done]
(1/1) Installing: workrave-1.10.44-3.2.src
...................................................................................[done]
timo@linux-sde2:~> ls -l /usr/src/
total 0
timo@linux-sde2:~> sudo mkdir /usr/src/packages/
timo@linux-sde2:~> sudo zypper si workrave
Loading repository data...
Reading installed packages...
Resolving package dependencies...

The following source package is going to be installed:
  workrave

1 source package to install.
Overall download size: 6.2 MiB. Already cached: 0 B. After the operation,
additional 6.2 MiB will be used.
Continue? [y/n/v/...? shows all options] (y): 
Retrieving srcpackage workrave-1.10.44-3.2.noarch                              
                                               (1/1)
Retrieving: workrave-1.10.44-3.2.src.rpm
.........................................................................[done
(3.3 MiB/s)]

Checking for file conflicts:
.................................................................................................[done]
(1/1) Installing: workrave-1.10.44-3.2.src
...................................................................................[done]
timo@linux-sde2:~> ls -l /usr/src/packages/
total 8
drwxr-xr-x 2 root root 4096 Apr 30 15:15 SOURCES
drwxr-xr-x 2 root root 4096 Apr 30 15:15 SPECS
timo@linux-sde2:~>