If a region has TNT and creeper explosions disabled, it would be nice if such explosions set off just outside the region were prevented from damaging blocks inside the region. As it is, people can successfully destroy the edges of a protected region in that manner.
I suppose an additional larger region with just those two things blocked (TNT and creeper explosions) could be placed around the target region to keep it fully protected, but that's a bit of nuisance to have to do. That, or just globally disabling TNT and creeper explosion block damage, but I'd prefer not to do that either.

I'm aware of this issue and I plan to fix it.