It is the file handler during the write that is inserting the null character before each tab and this is default behavior for line sequential files. In fact it will insert a null character before any non-printable character that is written to the file. This is because line sequential files should only contain printable ASCII characters. non-printable characters can cause problems when reading the file. They can cause premature end of record, conditions etc.
The null character is a way of masking the non-printable character within the file so that the file handler can safely read these characters in a record in which case the null will be removed.
If these files are only to be used as text files and you do not need to read them in again then you can turn off this null insertion by setting INSERTNULL=OFF in an extfh.cfg file or by setting the -N run-time switch as greiner posted above.
You can turn on the run-time switch by starting your application with (-N) on the command line or by setting COBSW=-N prior to starting your application.
example:
progname (-N)
or
set COBSW=-N
progname