See this video:
Video
AMYJ2OpZEzE

The blue portal doesn't fizzle if that specific part of its backing surface is removed; and a portal can be fired even if that specific part doesn't have a portalable surface.

Did I do something wrong? Or is this a bug?

Also, when both portals fizzle simultaneously, that's me fizzling them.

Thanks.

I know this would only be a workaround, but: have you tried parenting a func_portal_bumper with the size of the rectangle part you don't want to be portalled to the prop_wall_projector? This way it will come out from the wall tied to the prop, and will stand in the desired position avoiding you from placing a portal on its area...

Or maybe a func_noportal_volume instead.

Just give it a try and tell if it works; I hope it helps...

Anyway, to me this looks like a bug.

I recommend using a func_placement_clip.