Information from Budanova:
Tomorrow, Witkoff and Kushner will discuss with Putin the various security options for Ukraine that were discussed in the negotiations, and they will also discuss again in a tripartite meeting at the level of the leader, during which territorial issues can be resolved, since no delegation has the authority to resolve them. With Putin's answer, they will fly to Zelensky in Dublin.
This is the answer to why they fly to Moscow. Because Putin once again sent a delegation led by Medinsky, which has no mandate to discuss anything significant.