I am sorry, but your example has nothing to do with robbing or gaining player agency. First of all you just assume that the DM rolls again. A lot of DM would let the results of the scout count.
And a lot of DMs would roll another check. And plenty of games have rules requiring another check too.
But even if they don't and roll again, its so easy to justify if HOURS happen between the scouting and the actual travel. In fact it was a choice by the player to scout before being ready to travel. But none of that matter because its actually not the randomness that changes the encounter, its the DM changing the encounter. I can easily see how a DM does exactly the same with a pre-designed encounter, to change it based on the players decision to come back hours later. It has nothing to do with using random tables for this change or not.
That was not the question.
Having a pre-designed route or having random encounters has no causal effect on player agency, how do you present it and what consequences players actions have, these are actually affecting player agency.
the over given example of player agency is the player knowing a set encounter is at a location and then chooses to avoid it.
You can't "avoid a random encounter with player agency" as the encounter does not exist until the dice are rolled.
Huh? Wha?
Things can change. What you see on the bridge now might very well not agree with what's there two hours from now, and this is in no way an affront to anyone's agency.
I get it does not make sense....but we are talking about Player Agency.
Remember Player Agency requires both the game not make sense in a Reality Simulation sense and for the game to be a much more Rule Focused Game Play.
And, as you can see, plenty of pro Player Agency people, say a second roll should never be made. Their thinking is: once a location is "looked at" or "scouted" it must never, ever change. Though, sure, they will say a "reasonable time"....though that "reasonable time" will be whatever benefits Player Agency.
But then you won't see a lot of rolling on random tables in a high player agency game anyway.
This strikes me as a bit strange. Why would the PC assume that just because the bridge was empty once it would be empty hours later?
Player Agency. For the players to have any agency in the game things must remain static.
Now if they asked the local guides/guards/scouts if there were any monsters or bad guys spotted within a days travel (or did some other more thorough checking that would actually preclude the monster showing up), then that would feel different to me.
Note this would not matter if your rolling on a random table. Every NPC in the game could swear and sign an offical court document and say "there are no monsters at that location". But if you then roll a check and monsters are there: they simply appear out of thin air.
But otherwise, doesn't bad stuff essentially happen at random in real life the way folks often think about it? (Yes, the road home is usually safe, that's why we drive it. Yes there is sometimes a really unsafe driver on it anyway).
But, for a high player agency game you can't have "too much" of the "bad stuff" happen as then the players will feel they have no agency.