The execution of the $_internalOwningShard aggregation expression requires the fetching of up-to-date routing information in a context that does not allow to wait for the completion of an inflight cache refresh.
Such a logic should have to deal with situations where the returned CollectionRoutingInfo is stale or absent (this may happen in case $_internalOwningShard gets invoked right after the targeted collection gets sharded, which only triggers an asynchronous refresh of the routing info), but currently only the first condition is checked - and a tassert is instead hit when the latter one materialises.