As both of your comments aim at the same direction: I know that I make the setting my own and my version is important and "canon" at the table. I thought my example made it clear that the FR became pretty fast a version of my own. The problem is that my readjustments/improvisations/own internal consistency caused a lot of breaking points with the existing lore where I had to reevalute again.
So, no the problem was not that I tried to maintain accuracy with the wider world, the opposite was the case. But my own version caused me to break my line of thoughts 100x times, because it contradicted the official versions so I had to re-align everything that was stated in an adventure or in a description of a city, faction etc.
For example, early on, a player investigated a bit further into the Zhentarim. I didn't know too much about them besides some basics and so I quickly improvised some stuff about what I thought made sense. So my game was different from official lore in that regards. But now, everytime when I read something about the Zhentarim in an official city description for a city my players travelled to for example or in an adventure, everytime I read somethign about them I had to pause a moment and think about how to keep my version of the Zhentarim consistent.
The same happens in Eberron too of course, but in much lower frequency, because the setting is written open-ended. My Eberron is also quite different than the official one, but I stumble much less upon that fact when using official resources, because these resources were written with that expectation in mind.
I hope I could explain my problem a bit better this time.