Is it me, or when my seat gets shifted (through equipment changes or the latest CE faff) do I get the error message when I then try to then take change my seat assignment.

On a flight in a couple months where there is an equipment change myself and companion, booked on the same tickets have been given 15F and 11B with plenty of seats available in the cabin! The sensible change to make us sit together and I get the bleedin' error message