There are a few rules which make some sounds different and exceptional.
1. For Example, Double Zabar. While stopping at Double Zabar we read last sound as long as one ‘Alif‘.
2. Second example is ‘Alif’ itself. While stopping at ‘Alif’ (having one Zabar or two Zabar on it) the sound remains long up to length of one ‘Alif’.
3. Sometimes a few letters like ‘Daal, Waow and Meem’ have two Zabar and an extra silent ‘Yaa’ letter there, in this case sounds of Daal, Waow and Meem remain long.
4. The sign of Long Zabar exists in it while stopping and sound remains long.