sixpack 1.2 tape processing bug ?

Uups, tapes don't like me, I found a strange effect using a tape device/file by more than one user:  

- Hercules config tape 481 definition points to a WIN AWS file.

- Attach 481 to MAINT as 181
  MAINT's VMFPLC2 REW 181 works as designed
  DETACH Maint's 181

- Attach 481 to CMSUSER  as 181           
  CMSUSER's vmfplc2 rew     results in   DMSVMF110S ERROR READING 'TAP1(181)' 
  any other tape command also doesn't work

- After re-IPL and attaching tape to CMSUSER at first the tape file can be processed without any problem.

Bug or restriction that's the question :-)

Wolf