The MissingDependencies elements in the Solution.xml are a frequent cause of merge conflicts when developers are merging solution changes independently. In the event that you are using the split-missing-dependencies-steps.yml template to address this problem (as documented in the README.md), you will need to update the build process to ensure the unpacked missing dependencies are being repacked for the Solution Packager executes.
Move the Directory.Build.targets into a folder where all of your *.cdsproj solution projects are descendants. For example, src/solutions/Directory.Build.props (where your solution projects are nested under src/solutions).
If you are already using a Directory.Build.targets, copy the contents of this into your file instead.