It turns out there are two reasons for this, both stemming from the same piece of code in ImageFile.py

if self._exclusive_fp and self._close_exclusive_fp_after_loading:
    self.fp.close()
self.fp = None

If a file path is passed into Image.open() then it will be closed, but even if a file is opened explicitly it will be closed by the garbage collector after self.fp is assigned to None.

The solution takes three changes:

• In _open(), set __close_exclusive_fp_after_loading=self.is_animated
• At the end of open_rel, preserve the file pointer with self._fp = self.fp
• And in seek(), restore the file pointer with self.fp = self._fp