When analyzing a pull request or short-lived branch, a delta is computed so that issues are only raised on changed lines (the New Code being introduced)
For these analyses, the goal is to fixed the issues raised on New Code (and addressing other measures: like Coverage on New Code) until your code meets the standards in your Quality Gate, at which point you can be sure that you’re merging Clean Code into your main branch. I think we’re aligned on this goal!
(for technical reasons regarding how C# analysis is implemented, you will see issues for your entire codebase raised as warnings in your Azure DevOps build step)
On your main branch (or any long-lived branches) a full analysis is done that reports issues on all code, but breaks your code down into New Code and Overall Code. This is all in pursuit of our Clean as You Code philosophy.
So from what you’ve shown me – you have a branch/pull request analysis that has no changed lines (and therefore, no new code to raise issues on). If everything is working as expected, when you execute analysis on your Main Branch you should see issues being reported.