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?
Things like this are inevitable, especially these days.
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.