Two options. The first is to pre-compose the masked layer and then apply CC Page turn to the pre-comp. This will let the effect extent beyond the edges of the mask.
To keep the effect contained in the area of the mask spin down the CC Page turn effect in the timeline and click on the little + button next to compositing options, then select the mask you want to use. This will limit the effect to the masked area but it will also turn off the mask so you will end up with a hole in the layer with the page turn effect inside the hole. You can turn the mask back on but then the effect will be limited to the mask area so I may not look the way you want it to look.
I tested the second option which was I needed, but the hole you mencioned actually makes the effect happens in a different way other than my needs. Maybe this is an issue related to the fact TURN PAGE is ancient effect since LINE SWEEP works perfectly with a masked area.
Another option is to duplicate the mask layer then use it is a track matte for the layer with CC Page Turn applied.
Did you try Rick's option for duplicating the mask layer and then using it as a track matte using CC Page Turn? Please let us know if that worked.