We use to worry about that too. Now we omit the moisture barrier entirely. We’ve found it to be harmful because: 1) it’s useless, 2) inhibits the curing process, 3) creates moisture problems later
1) I’ve never had anyone explain how a moisture barrier is suppose to work when the contractor drives grade stakes through it 10’ on center.
2) if it rains the night before a pour or there is Dew in the morning, moisture will lie on the top of the moisture barrier in pockets. This moisture will cause the slab to cure longer on the bottom than on the top. This requires all the moisture to come out the top of the slab only. This differential “drying” will cause the slab to shrink differently from the top than the bottom.
3) When (and if) moisture comes up from below later, it will seep in around the grade stake holes and then lie on the bottom of the slab. Then the moisture will be drawn through the slab ruining any floor finish. Without the moisture barrier, it will recede when the water table goes down. With a moisture barrier it’s trapped there until it can evaporate through the slab.