This came up in one of the Ravenloft threads and I am just curious: do you care about official aka "canon" lore for D&D, either the implied setting or a specific campaign world?
If a DM is using a specific setting, I assume it is because they want to use that setting, and that it's shorthand for the common understandings of that setting. So I will, as a player, heave to that lore. If we're playing in Eberron and I base character assumptions on The Last War or the Dragonmarked Houses and they aren't represented, I'd be put out.
I mention as a player, as a DM I almost exclusively run homebrew and give my players a lot of narrative say, including in the lore. (Oh, and I do are about that lore.

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As a side note, it's one of the reasons I'd never run FR with one particular group. It was a bunch of people who do read the novels and are very familiar with the Realms. They knew the lore of it much better than I and I would never be able to do justice to it being "The Realms" - it was much better to run homebrew or another world instead of constantly tripping them up with things they knew but were false in my Realms because I didn't.
Does it bother you if that lore is changed with editions? Should a new version of a setting be "required" to not contradict a previous version?
A bit of the opposite - if the mechanics change in such a way that no longer supports the old lore, we need to change that lore. If new things are added to the system, such as new common playable races, that weren't supported in setting lore thought should be given to if they need to be lore or excluded from the setting. (I have no problems settings modifying core assumptions, including like Dark Sun excluding races and classes and adding their own.)
For my part, I don't care much at all. Chances are I am going to change some stuff anyway if I am using a published setting and if I am homebrewing chances are the stuff in the Monster Manual or whatever isn't relevant in the first place. I don't read novel lines or pour over setting books, so I probably wouldn't notice most changes anyway.
If you run a published setting and change lore, please tell your players. I don't know why you mentioned the MM, that's not likely lore - change monsters around mechanically as you see fit. But if they go in with misleading lore assumptions because you told them "I'm running Dark Sun" but not "I'm running Dark Sun but the sorcerer kings have been defeated and arcane casting is culturally acceptable" then it's on you.