Nether portals break bedrock

[Kedarin]

Hi,
I have problem, on my overload the bedrock are flat only at the Y: 1
When my player mine a complete chunk and create a portal ( in the overload ) they take the portals ( now they are in the nether world).
After they return in the overload they break the portals and when a player take the portals in the nether the portals replace the bedrock.

Have you a solution for fix that ?
Thank's

[Kedarin]

I'm in 1.8 so I use multiverse 2.5