Hi OneBridge Solutions Questions,
The general steps are as follows:
lead/opportunity -> Summary tab -> Stakeholders -> Three dots in the upper right corner -> + New Connection

There is very little chance of making a mistake in the steps, you can switch browsers or try to add another user as a stakeholder.
The stakeholder subgrid will only display contacts for the corresponding role.
Please check the Connection tab to see if the contact you just added is there and if the role is correct.

Next, troubleshoot the possibility of plugins and custom workflow.
Try adding the stakeholder again with the System Administrator role.
If the above steps do not fix your problem, I suggest you raise a support ticket:
https://admin.powerplatform.microsoft.com/support