I've just managed to consistently reproduce the issue and it seems that its a Galaxy S25 Ultra issue, although the TP-Link router could be a factor also.
Steps to connect to Wi-Fi 7:
1. Connect to the MLO network on the phone.
2. Restart the phone while connected to the MLO network.
3. Log in to the phone.
You will then see the Wi-Fi logo on the top right with no symbol of Wi-Fi 6 or Wi-Fi 7, just the plain Wi-Fi logo. This means I'm connected to Wi-Fi 7 MLO. Proof? In WiFiman, it'll say that I'm connected to both 2.4 and 5Ghz with MLO enabled.
The issue happens when I disconnect from the MLO network. It'll become a Wi-Fi 6 network and when I reconnect, it won't be back to Wi-Fi 7 till I restart my phone and do the steps above.