Currently, I am with employer A and transferred by H1 to employer B.
The transfer with B is completed. Now I don’t want to join employer B
and continue with A. From my side, I submitted I-9 to Employer B and
not join Employer B. Is there any problem?
No issues, you can keep working for your current employer. Unless you start the job, employer B can’t file the I-9.
At some point employer B will withdraw your H1B because you didn’t join them however this will not impact your H1B with current employer A.