The frustrum issue should be fixed in the coming Sweet Home 3D 6.0.

Actually, I discovered that most graphics cards (maybe even all?) support a Z-buffer of 24 bit, therefore I forced the Z-buffer bits to 24 instead of the default value of 16, and this nicely resolved the issue.
It will even optimize a little the rendering speed when the point of view changes, because less computing is required with a larger Z-buffer! [:)]