The tassert:

https://github.com/10gen/mongo/blob/35c2a0687bc4437c3408dc71a7073d1d0210fa00/src/mongo/db/shard_role/post_resharding_placement.cpp#L31

Resharding fields info is not local info, anything trying to access this must be up to date. The CRUD ops are versioned so they are fine. The command used to be versioned but was changed by SERVER-128052. PostReshardingCollectionPlacement is not also not using the LocalReshardingOperationsRegistry (SERVER-131499), which would avoid the need to even look at the resharding fields. However this requires the feature flag to be ON (default on, not gated by fcv).