Okay, I think I get where the miscommunication is here. You're thinking about what the DM should just tell the PCs based on a monster knowledge check, while I'm thinking about what the PC can figure out based on the results of their actions.
I have no problem with the players finding out reasonable game mechanics information as they progress through an encounter. I'm merely talking about what the DM describes when the encounter starts.
A player gets a 22 total to hit against AC and misses, the player knows that the AC of the foe is 23 or higher. I have zip problem with this level of gradual and progressive player information gain.
Let's clear things up with an example:
PC Warlock attempts to use Rod of Reaving's property on Monster X, but observes that it has no effect. He has previously observed that every time he uses RoR on a monster and it has no effect, that monster ends up going down in one hit.
PC tells his party: "Okay guys, I just used RoR on Monster X with no effect. That means he must be a one-hit monster, so don't waste your big guns on him!"
Is there anything metagamey about this line of reasoning, and if so, what?
You find this reasonable?
I would never tell a player that the Rod of Reaving had no effect on a minion. For that matter, I would never tell a player that the Rod of Reaving had a 1 hit point effect on any other monster.
I would just mark a hit point of damage on my DM sheet for either creature and move on.
A single hit point??? Are you kidding?
You find it reasonable that a Rod of Reaving should be a Minion Divining Rod? WotC changed the Rod so that it was no longer a Minion Killing Rod. I hardly think they did that to change the Rod into a Minion Divining Rod. That's an accidental side effect of the lousy minion rules and the fact that they had to nerf the Rod. It's not intentional.
Sorry, but if a player brought such metagaming nonsense to my gaming table, I'd tell him where the door is.
Course, I doubt I would ever hand out a Rod of Reaving to begin with (it's a pretty bland item), so it would never be an issue at my table.
There are always going to be unintentional side effects with regard to certain items and player knowledge. That doesn't mean that the DM has to hand out game mechanics information because certain rules (like the minion rules) are inferior.