In "legacy" .NET Classic Framework, using NuGet reference format packages.config, NuGet will automatically update the .csproj files to import .props files included in the package. Dependabot does not follow this behaviour currently, leading to broken imports - the import is left as it was, pointing to the previous version of the package.
Sample package using this behaviour: https://www.nuget.org/packages/Microsoft.Net.Compilers

Sample diff when upgrading via Visual Studio:

Longer explanation how the .props files are handled - on Stack Overflow
relates to #541
I've just realised this is more serious: the updated for projects using packages.config are currently completely broken. This is because those projects have a <Reference> with <HintPath> in the project file that needs to be updated at the same time - and right now Dependabot doesn't do this.
See the sample diff how it should look:

Dependabot only generates the second part, ignoring the project file.
This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs within seven days. Thank you for your contributions.
@stale this issue is still valid and breaking updates.
We are also seeing this on our projects.
Here's an example where I ran into this. I keep having to add my own manual commits to make the build work
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We are also seeing this on our projects.