This has been the case for a long time, probably since forgotten attacks started. My guess, and it is only a guess, is that a sub-routine is started when the forgotten attack and once started no further check is made to see how many waves or even if the forgotten base exists.
It is also the case that if an attacking player base is destroyed during an attack it continues to the end and doesn't stop despite no base existing.
Both events are probably best described as a 'feature', which in plain English means that it is a bug that isn't likely to be corrected.