When completing airlaw questions just read the law as written and apply it; as stated above the rule states that if using area QNH 50ft must be added to all minima.
12.17.3 Up to four forecast values of QNH are given, for the times HH,
HH+3 hours, HH+6 hours and HH+9 hours, where HH is the time
of commencement of the TAF validity period. These forecasts are
point forecasts for these times but are valid for: in the case of the
first value, ninety minutes after the time point HH; and, for
subsequent values, ninety minutes each side of the time point.
The QNH forecasts are prefixed by the letter “Q”.
Ando1bar;
If you were planning to a destination with a 12 hour forecast and planned to arrive 65 minutes before the TAF expired you would be outside the 90 minutes from the last QNH provided. Therefore the planned arrival would need to be on the forecast area QNH, add 50ft to alternate minima. You still have a valid forecast but not a valid QNH at arrival time. It is a very unlikely situation however it is remotely possible.