I made a booking for travel last week.
It included flights yesterday and this Friday.
For the Friday flights, 5C was preallocated for both segments (73H). Nothing for yesterday's (although at OLCI I snared 4D).
This morning I clicked on the
Select Seats icon.
For the first flight this Friday it showed my as being in 5C - 4C/D were available, I selected 4C and clicked next.
It then showed me as being in 5C for the second flight; 4C was blocked, but 4D was 'available' I selected it and clicked Next.
I then was shown a screen with an error message as noted, 4C for my first flight was displayed; it only indicated forward for my second flight.