Not able to get line-by-line hits

[oloapinivad]

Describe the bug

We have an open source repository where we have set up the codecov coverage. Overall, the coverage report looks fine https://app.codecov.io/gh/DestinE-Climate-DT/AQUA, but when we go digging to the single file to check the line by line coverage I alwasy get the following message

There was a problem getting the source code from your provider. Unable to show line by line coverage.
If you continue to experience this issue, please try [logging in](https://app.codecov.io/login) again to refresh your credentials.

Of course, the repo has right authorizations, and re-logging does not help. I tried to patch up the codecov.yml in different configurations but I still have this issue. I also tried to modified the fixes yaml block since apparently there was a mismatch in the structure of the file, but it does not appear to solve either. I am checking the reportin the coverage.xml but cannot find any useful tip.
The actions as well does not show any evident error.

Any hint on what I have configured wrong?

Environment (please complete the following information):
Running on GitHub actions, Python environment with mamba

Expected behavior
Getting the line-by-line hit

Screenshots

Additional context
One peculiar thing of our repo is that we do almost never run tests on the main, but rather at 99% we run tests on the PR and the merge without running them again to save minutes. I noticed that codecov seems to not "inherit" the coverage from the commits on branch which are merged, but only if tests are run on the main. Not sure if this is the same problem or a different one.