Unable to route data between browser nodes

[ShookLyngs]

Running 2 helia nodes in browser and a relay server in Node.js. The whole relationship looks like this:

A <-> Relay <-> B

A: libp2p + helia, running in browser, connected to Relay but not B
B: libp2p + helia, running in browser, connected to Relay but not A
Relay: libp2p, running in Node.js as a CircuitRelayServer, connected to A and B

After I added some directories and files in A's MFS, I cannot find them from B's routing system:

const cidFromA = CID.parse('...') // a CID from A's MFS
for await (const provider of bIpfs.routing.findProviders(cidFromA, {
  signal: AbortSignal.timeout(10_000), // 10 sec timeout
})) {
  console.log(provider.id, provider.multiaddrs)
}

// libp2p:kad-dht:query:QmbyXqDhMejQJjcq6nGJBYGciyUmJYRVmtgv1Y23phjZN4 
// routing table was empty, waiting for some peers before running query +0ms

// after 10 sec: signal is aborted without reason

Whether A has a direct connection with B or not, the result is the same.
The only difference I found is when A and B are connected, the mfs.cp() method works:

const cidFromA = CID.parse('...') // a CID from A's MFS
await mfs.cp(cidFromA, '/<path_in_b_mfs>') // successfully copies the file/dir from A to B

I wonder if this is expected in ipfs and helia?