I worked with some other outsourcing companies from other part of the world, and seen exactly this pattern: The initial team was sharp and smart, to gain stakeholder confidence, they stayed for a few months, setting up demo and prototype. Later they would slowly bring in other team members and started to withdraw themselves from the project. Eventually, those A-team was gone.

Those late-joiners had absolutely no clue on what was going on, and also didn't have the competency at all to hold the helm. Result? A much mediocre rewrite of the whole platform which never fulfilled the requirements.