How much you can get away with reskinning depends on how much you reskin and what exactly it was the players wished to avoid.
I think the bolded bit is crucial.
Eg if the PCs are hoping to avoid ogres because they have been cursed so as to be peculiarly vulnerable to ogres, and they successfully perform actions to try to hold off the effect of the curse, then springing an ogre on them seems like negating the consequences of those actions. That seems pretty railroad-y. But if the opponent is a giant mace-tailed lizard that just happens to closely resemble an ogre in stats, well that doesn't seem railroad-y at all (in this context).
Contrast the PCs hoping to avoid ogres
and other similarly dangerous encounters because they're low on hit points. Suppose they have successfully performed actions that have revealed to them that ogres are the dangerous things around here. And suppose they've cast
invisibility to ogres on themselves. In those circumstances, just making up the presence of a dangerous mace-tailed lizard that the PCs just happened never to learn about while doing their research into local threats seems like it might be pretty railroad-y.
Trying to diagnose railroading without coming to grips with
what is at stake and
how the GM's manipulation of the fiction relates to that is silly.
Yeah some players are never going to enjoy exploration under these circumstances.
Although part of the art in doing this is working out as you go how it all interacts with every else that has already been established.
I guess it's possible to do an exploration-oriented hexcrawl with on-the-spot generation of hex contents. But it's probably not ideal, given that it really limits the capacity to give the players the sort of information that might let them make reasoned decisions about which hex to explore next.
But most generate-content-on-the-fly RPGing is probably not exploration oriented. I did this all the time GMing 4e D&D. But it's not to support exploration - it's part of framing situations that will engage the stuff the players care about. (Which in my games isn't normally the map.)
As I mentioned far upthread, some of this may lead to problems though, if you really are (in some sense) "making the world exist" only in the moment it's called for. A player felt there was no tension, no weight to choosing, if they were choosing Currently Faceless Void #1 vs Currently Faceless Void #2, when I would roll and whatever I would roll would end up being in whichever thing they chose.
This seems like a problem only in the sense that I find RPGing which prioritises mapping - and minutiae of architecture more generally - unbearably tedious, and hence old school dungeon crawling may lead to a problem.
I mean, we all have stuff we don't like, but that's not really a problem with that stuff.
Problem carries normative weight that just isn't apposite in this context.