This seems to be due to the fact that the series is pre-selected in auto-name for packing slip:
So if you want to select a different series, this will be prevented by the code itself.
This is unlike the sales invoice where the naming is not “hard-coded” but is picked up from the naming series selected:
Ideally, if auto-name is tied to a series, the system should not show the selection in naming series as it will cause this issue.