Even in a product as technically wonderful as Temporal, we can have relatively simple oversights like this that lead to cross tenant leakage.

If anyone is more familiar with Temporal, is there a way clients could have had internal defense in depth that guards against tenant leakage at the provider (Temporal) level?

Don't use namespaces. Wire up multi-tenant at the RBAC level. Need stronger isolation? Run another cluster.

Encrypting tenant data with per tenant keys is a good defense against this kind of thing.

Things like this are inevitable, especially these days.