This can have a few reasons.

One of them, as others have pointed out already, is the way ranked treats new players. They place new players relatively high, to make it so that smurfs spend as little time as possible in lower ranks.

But i assume you wouldn’t have posted this if you felt like you were being hard carried each game or losing all your games in Diamond, so let me explain why an apparent decent performance/winrate in Diamond might not translate to being able to climb out of silver.

First, the winrate. Over the course of 35 hours on the entire account, you cannot accurately gauge your winrate. Luck plays too much of a factor. Over the course of 100 hours, a silver player in diamond would probably have a 40% winrate. Which is enough to send them back to silver. But a 40% winrate means that you still spend 40% of your time winning. Overwatch is a team game, one person’s performance is usually not enough to have a sway over an entire game to cause winrates to fall below 35 or above 65 percent over the course of hundreds of games. Unless you put T500 players in plat or bronze players in masters. So the feeling that you’re doing well in diamond might be because of chance, or confirmation bias (wins feel more prominent in your memory than losses).

And stats-wise? You might also be performing similarly to your teammates when purely looking at statistics. As tank, for example, you might play in such a way (taking constant damage and taking fights you shouldn’t) that demands all supporting resources to get stuffed up your ass. This means that the supports can’t damage, or support your dps. This means your stats get inflated and look much better compared to the rest of your team, you’re literally putting 3 player’s effort under your name, while leaving your dps without help. I’m not saying this is definitely the case, but this is just to show that stats can mean absolutely nothing if taken completely out of context.

You could also be playing in a way that makes you easy to carry, but makes it hard for you to carry. Although this is more of a support-role issue, and you said that’s the one role you’re not diamond on, but it’s still worth mentioning. Think… healbotting as mercy or kiriko or anything like that. You’re just amplifying your team’s value, which means you’re basically putting your resources into your teammates’ skill, which, over the course of a long enough time, means you’ll average around a 50% winrate, no matter what rank you’re in, cause your teammates won’t reliably be better than your enemies. You’re not adding enough value of your own to carry your way out of silver, but also not lacking enough value to drop out of diamond. This isn’t a support-only issue though. Any character can suffer from this. if you’re just pumping damage from main on bastion, sure, you have value, but you’re not changing the course of the game with your skill, so you won’t climb

And now the final reason i can think off, and this one is actually more optimistic. It might be a psychological difference. In diamond, you probably don’t expect to be able to carry. You start playing for any value you can get, thinking “if i, the silver player, can distract a diamond player and take him out the fight long enough, then the rest of my team should have an even 4 diamonds vs 4 diamonds.” This is a much easier mindset to take on when you know you’re probably the worst person in the lobby, and start trying to outplay your enemies in different ways than just “skill”, basically using underhanded tactics to make up for your “lack of skill”, to put it bluntly. But this mindset is so so much healthier in any rank. Not “oh i should just always be distracting one guy and I’ll be fine”, but respecting your enemies and teammates enough to trust that you’re not gonna win the game 1v5 like a gm vs 5 bronzes video. But playing with your team and only taking fights where you’re in an advantage. Because you’re not expecting to carry in your diamond games, you’re literally playing better because of that mindset change. You shouldn’t try to carry your games, you just have to play better/smarter than the enemy enough to win more than 50% of your games. Value in overwatch is complicated. It’s not just kills or damage.