Blocks placed in Panel Pages behave erratically

When attempting to place user created blocks within a Panel Page layout, I ran into problems, the most obvious of which is that any blocks I've placed eventually disappear from the panel. After some experimentation I've been able to come up with the series of steps that will consistently reproduce the problem. (See the last 3 steps for where the problem occurs).

To be sure, I tested this with a fresh install of Drupal 6.16, Panels 6.x-3.3 and CTools 6.x-1.3, and no other modules installed. All configuration are at default values, and I was using Garland. Please let me know if I can provide any more info, (especially if the test case below doesn't give the same results that it gives me).

• Create 2 Blocks, "Test Block 1" and "Test Block 2".
• Add a new Panel Page (/admin/build/pages/add)
• Any layout will do, for simplicity let's choose Single Column.
• Click Finish, then click Save (leaving the panel empty for now).
• You should now be on the Panel Page Summary, return to 'Content'.
• Add "Test Block 1" to the region.
• Click "Update and save".
• Add "Test Block 2" to the region.

  This is where I start to experience an issue. "Test Block 1" has been marked as modified, even though it has not been changed.

• Click "Update and save".

  There are now two copies of "Test Block 2", and "Test Block 1" is gone. (Looking at the
  actual page at this stage confirms this is actually true, and not just an error in this view).

• Still under "Variants > Panel > Content", make any other change.
  (For example, just set the title of the Panel).
• Click "Update and save".

  Both copies of "Test Block 2" dissappear entirely.