NBXplorer: Broadcasting BCH/BTG Tx fails with Signature must use SIGHASH_FORKID

builder.SendEstimatedFees(feeRate);
var tx = builder.BuildTransaction(true);
Assert.True(tester.Client.Broadcast(tx).Success); // <- FAIL

This fails with:

BCH: Transaction 3d13e1a11893ba832319eef1a0c77d397dc8cc7c8c8e7a22dd3c91552c277615 failed to broadcast (Code: RPC_TRANSACTION_REJECTED, Message: Transaction was rejected by network rules, Details: 16: mandatory-script-verify-flag-failed (Signature must use SIGHASH_FORKID) )

Please, what could cause this, and how can I resolve this?

The same issue is true with BTG network.

Related: https://github.com/dgarage/NBXplorer/pull/50