My own experience was that in Y on ARN-LHR last week (my second data entry, but the other was on an 99% full flight so not relevant) I did not get seat block at all at my exit row despite there being in the order of 6-7 spaces in the rows from 5 to the first emergency exit row (13? on the A320).

Luckily enough I haven't flown any other Y segments since FLY was introduced.