Darshan is building his next version of slideshow component and this problem is bugging him now.

I found out one workaround yesterday. When the mask movieclip made open (instead of keeping it with a closed hole) even with a hairline gap makes the mask work properly. Hiding the hairline gap we created is another trick. I will write about it later

Today I found another trick. When your mask is based on a rectangle with a hollow space inside you can use the following technique. We just need to change the sharp top left corner to rounded corner or cut out a triangle portion from the top left corner. Then it masks properly