Is gas required to execute a transaction not deterministic?

Hi, I was trying to deposit to Gemini today and ran into an interesting problem. About 270 days ago, I was able to deposit into the contract address they gave me with the standard 21000 gas.

I tried again today with the same 21000 gas limit, and the transaction ran out of gas. So then I tried using my wallet's auto gas limit calculation thing and it gave me a number pretty close to 21000, so I just assumed there might've been some changes to the network (e.g. some EVM instruction costs more gas than it previously did to run). I sent it with the recommended amount and it failed again. I'm currently waiting on my third attempt (with an even higher gas limit) to confirm.

Can anyone provide any insights as to what's going on here? The contract obviously hasn't changed since the first time I sent ETH to it, so why does the same gas limit not work (and why didn't my wallet set a high enough gas limit when I set it to auto?)