My impression from the movies was that the ring WOULD grant power to someone who sought to use it for that purpose, if for no other reason than to make itself even more precious (ha) to the wielder. In the case of Isildur (who, as a Man, is implied to "above all else, desire power"), the Ring knew it was in a bind. Sauron just got yeeted into the shadow dimension, and it was in the one place it could be destroyed. It NEEDED Isildur to get him out of there, pronto. So, if Elrond decided to force the issue, my assumption was always that it would lend Isildur some fraction of the power Sauron put into it as matter of self-preservation. Furthermore, by granting Isildur a taste of its power, he could fall even more under its control because power is what he wants. More self-preservation from the evilest bit of jewelry ever forged. Finally, by the time this hypothetical confrontation between Isildur and Elrond would occur, Isildur has already fallen to the Ring's temptation and rejected the call to destroy it. So, the Ring may not have any qualms about strengthening Isildur, as doing so would not endanger it at all.

Now, to contrast with Frodo (and Bilbo, to some extent). They do not desire power, and so the Ring's biggest bargaining chip is rendered useless. Additionally, for 99.9% of the journey Frodo maintains his resolution to destroy the Ring. Granting power to Frodo at any point prior to those last few steps would be a risk to it's existence.

It's been a very long time since I read the trilogy, so I don't know how much of this interpretation conflicts with legitimate explanations Tolkien gave in the text or in his correspondences, but it works for me within the context of the films.