The occasions where you get to start something from scratch are few and far between, so buckle up.
Digital products—especially the ones with actual users!—are like living organisms, always in some kind of flux. It is very hard—read: laborious—to keep all the things in order, all the time.
Why you may ask? Well, because pressures. Designers are constantly pressured to deliver BUDs—barely usable designs—so the work can move on towards implementation. Details be damned, we'll figure it out in production. This is why we have QA after all! I hope you can sense the sarcasm…
In these real-life circumstances things are left in states which are… let's say not perfect. This is what we call design debt, and guess what: sometimes you will be the one who gets to work with it, or sort it out.
It's part of the job. Get used to it.