@sorawee Suppose we store a hash table in a weak box. Then the hash table is reachable through weak references only.

If the hash table is weak, then a value stored in the hash table is reachable through weak references only, and the garbage collector is allowed to replace the value with #f.

If the hash table not weak, then a value stored in the hash table is not reachable through weak references only (even though the hash table is stored in a weak box). Therefore the value can’t be removed by the garbage collector.

So the answer to your question must be no.

However, if the hash tables aren’t stored inside weak boxes (or weak hash tables), I think, your observation holds.