Anything that cares about the zeroth IP of a destination not on the local subnet is broken. Anything that caters to broken is worrying about the wrong things.
I've spent the past two weeks just trying to fix some horribly wrong/invalid data that is absolutely positively guaranteed to always be correct due to certain standards and lots of money and one of the biggest best tech companies in the world stands behind it.
Guess what it's still broken they just refuse to admit it.
"be conservative in what you do, be liberal in what you accept from others"
And there's no takebacks -- once you start permitting something, you can't take it away later, or you'll break things. You're creating more legacy requirements that may cause you problems when you try to evolve in the future.
As a web developer, I can only dream of how clean web standards might be today if the browsers of yore were a bit more conservative.