The two Infinity Gauntlets, from the post-credits scene in Avengers: Age of Ultron and Avengers: Infinity War, do have some physical differences to them. This would mean they are likely not the same Gauntlet. However, from my answer here we see that the post-credits scene probably took place on Nidavellir.
The Avengers: Age of Ultron post-credits scene saw Thanos claiming the Infinity Gauntlet and saying, "Fine, I'll do it myself," after Ultron had failed to wipe out humanity. However, in Avengers: Infinity War, it was revealed the Mad Titan had forced Eitri to forge the gauntlet for him. With Eitri's Nidavellir home having been under the protection of Asgard, many fans claim it's impossible for him to have given Thanos the gauntlet so long ago. Joe Russo, director of Avengers: Infinity War, has explained to ComicBook.com that the post-credits scene did, in fact, take place on Nidavellir.
"I think that it would be connected to Eitri," Joe Russo said. "I think that clearly he is the one who forged the gauntlet and Thanos had the gauntlet at that point in time. It's been a while since any of the Asgardians have interacted with Eitri and his people."
ComicBook, 'Avengers: Infinity War' Directors Explain That 'Ultron' MCU Plot Hole
As such that gives two probably, very similar, possibilities to explain this:
The Gauntlet wasn't quite finished or needed some ever so slight touching up after the scene in Avengers: Age of Ultron before we see it in Avengers: Infinity War.
The Gauntlet we see in Avengers: Age of Ultron is actually one of the prototypes that Eitri makes.
Of the two I think the second is probably the most likely. After all, we do see a prototype Gauntlet in Avengers: Infinity War when Thor, Rocket, and Groot all initially get to Nidavellir.
Out of universe, though, of course, Thanos was re-designed himself ever so slightly for Avengers: Infinity War and so refining the Gauntlet was probably also part of that as explained by "Matt Aitken, VFX supervisor on Infinity War for Weta"
"Those sequences in Guardians is where we see him most clearly in the previous MCU films. It was a wonderful freedom to not have to match that exactly. I think it's one thing to have a Thanos who in those scenes was definitely more stylized, holed up for those short beats that he was in those films. But when he had to be the lead character and hold film and appear in hundreds and hundreds of shots, it was great to have the latitude to refine his look, to make his facial appearance more natural, so he's definitely more detailed and has a more natural shape to his face, to his lips, to his brow, his nose, and all those details we're talking about in terms of the blemishes and the pores and the stubble. If we'd been confined to those earlier iterations, it would have been too much of a limitation, I think, so that was never an issue. Right from the start, Marvel recognized the need to go to the next version of Thanos, and we were never asked to match that earlier version at all."
Screen Rant, Why Thanos' Design Changed So Much For Avengers: Infinity War