Is there a special significance behind the seven alifaat being pronounced when stopping, even though the other alifs with the same symbol are silent, or is it just because Hafs read that way ?
There is a difference between the "zero" on the alif for the seven alifaat and the "zero" symbol in which the letter is never pronounced.
When there is a round zero -called الصفر المستدير , the alif or other letter with this symbol is not pronounced.
When the "zero" is an oblong one الصفر المستطيل القائم, it is pronounced when stopping but eliminated when continuing. This is the type of symbol in the seven alifaat.
In the end, it is riwaayah, or the way it has been transmitted to us for the recitation of Hafs (other qira'aat have different ways in this case), but the symbol gives us the indication of how we pronounce it when stopping in comparison to when we continue.