You’ve got split flow optimization turned on, which is good, however, RAS is not able to converge on the flow distribution within the number of allowed iterations (30 by default, 60 max…I think). You can try to up the max number of iterations for split flow, or adjust your initial “guess” at the split flow. There are many other things that can affect this. Check all of the geometry inputs, especially in the area of the split and downstream for errors. Make sure you have enough cross sections. If you’re losing a lot of flow through the breach, optimizing on the flow split could be difficult to do.
Also, why are you not doing this in unsteady flow? A levee breach is typically done in unsteady flow because of the significant attenuation impacts of the off-channel storage, as well as the dynamic (i.e. changing over time) nature of a dam or levee breach. Unsteady flow will also do a better job of determining the flow splits.