"Why is Riot hurting my Off-builds when it's the main build that's the problem?"

Champions will always have "True Builds" (The build that follows the design the champion was made under, and the most commonly built build by the community). was designed under building lots of AP, and since the most commonly built build by the community is Full AP, that is his "true build"

"True Builds" exist because of how a champion's kit works. Would you want someone with as much CC as Naut, having top tier DPS of a Fighter? If such a thing existed, and became problematic, the Tank Naut would be preserved over the AD-PS (AD DPS, get it?) as it's the one they've deemed the original concept and healthiest.

This is what matters most when it comes to "True builds" and Off Builds, and how balancing occurs to each. For example, let's use the AD Poppy offbuild. Poppy's original concept is a disruptive tank that likes to stick a hammer in your face. Due to the concept of a tank, she is meant to have a over time kill time pattern when not completely ahead, and as such, her Q short CD, Anti-mobility and bonus resistances amplify that idea. Poppy's nerfs were targeted at her damage from her Ult and her shield duration.

Now the complaint goes "Why are you nerfing my off build when it's the main build that's problematic?" The simple answer is simple: Your build is a joke. Not to be rude, but off builds tend to be based off some sort of gimmick. For AD Poppy (assassin/high damage diver), the idea is to burst your opponent while they're stunned, knock em up, and finish them quickly. (You're trying to act like the Old Poppy essentially)

This is where the issue comes in. People enjoy playing these offbuilds since they are fun, but they are, when it comes down to it, Joke builds. They are meant to be a build to have fun in, they aren't serious. That's why True Builds are balanced without consideration to off builds, since they aren't abiding by the way the class was designed to be healthy with and are simply joke builds. (Hello Tank Assassins and AP Yi) Since Poppy was outdueling people with her high shield uptime with CoC and insta-cast Burst/stall for CD Ult, it was those that got targeted despite the existence of an AD Off build.

A great example of an Offbuild that Riot actually encourages because it DOES add serious variation of a build path other than a joke, is AP and AD Ezreal. Both accomplish a role, with different qualities to them, and they are separately capable of being balanced due to ratios. The difference strikes with Poppy, is that she is meant to be a tank who does some pretty decent in your face damage, but that quality overlaps with AD Poppy, which is primarily in your face damage, but less tank.

TL;DR: So when Tank Poppy is doing a bit too much damage, what gets hit is the same quality that AD Poppy wants, which is unfortunate but the off build is incapable of being separately balanced without the main build losing what makes it special, special.