Testnet queries return 'failed to obtain coinbase address' inconsistently

Hi all,

When using the cronos-testnet RPC
`https://evm-t3.cronos.org` or
`https://cronos-testnet-3.crypto.org:8545` and `wss://cronos-testnet-3.crypto.org:8546`

We receive the following error intermittently (between 50-70% of the time depending on what we query).

`{
code: -32000,
message: 'rpc error: code = Internal desc = failed to obtain coinbase address: failed to retrieve validator from block proposer address : validator does not exist'
}`

we are using the Web3 JS library to handle RPC calls, the types of RPC calls that fail are just simple contract reads such as token balances etc.

We also find that when using the testnet returns incorrect blocknumbers at times. It will consistently return incorrect block numbers of either 5925946 or 5957848 when `using web3.eth.getBlockNumber()` about 70% of the time as well. I tried this with the mainnet RPC URL and of course there are no issues there.

Any solutions/causes to this issue or any ideas where this might go wrong client side?

Thanks!

Reproduce the intermittent failures against the listed Cronos testnet RPC and WebSocket endpoints using Web3 JS, starting with web3.eth.getBlockNumber() and simple contract reads. Compare returned block numbers and error responses across endpoints; done means identifying whether the inconsistency is in the testnet RPC service or the client-side calls.

Written by the indexing model from the issue text.