I believe this is a bug introduced in 9.0. When the source index is deleted, the ResizeAllocationDecider throws on null index metadata due to the usage of projectFor. But the code is mean to handle null indexMetadata as indicate by the null check right after the line. Indeed, the version in 8.19 handles null properly.

Throwing exception on null can break desired balance computation and effectively stops the allocation process. We should fix it by handling deleted source index properly.