A user reported to me that they are unable to create a PSBT with a legacy watchonly wallet when they use the GUI. They get an error "The amount exceeds your balance".

I think the issue is that the GUI will retrieve the wallet's balance via `AvailableCoins` before calling `CreateTransaction`. This call to `AvailableCoins` is resulting in fewer coins being returned with recent changes to how `AvailableCoins` works, particularly with additional filtering of spendable coins and preset coins.

***

Note that this does not effect descriptor wallets. A workaround for this issue is to switch to using descriptor wallets. An existing legacy wallet can be turned into a descriptor wallet using the `migratewallet` RPC. However many watchonly legacy wallets are likely the result of importing descriptors via `importmulti`. For such wallets, I recommend creating a new descriptor wallet and re-importing the descriptors.
{
    
    TestChain100Setup test;
    for (int i = 0; i < 5; ++i) {
        test.CreateAndProcessBlock({}, GetScriptForRawPubKey(test.coinbaseKey.GetPubKey()));
    }
    auto wallet_loader = interfaces::MakeWalletLoader(*test.m_node.chain, *Assert(test.m_node.args));
    test.m_node.wallet_loader = wallet_loader.get();
    node.setContext(&test.m_node);
    const std::shared_ptr<CWallet> wallet = std::make_shared<CWallet>(node.context()->chain.get(), "", CreateMockWalletDatabase());
    wallet->LoadWallet();
    wallet->SetWalletFlag(WALLET_FLAG_DESCRIPTORS);
    {
        LOCK(wallet->cs_wallet);
        wallet->SetupDescriptorScriptPubKeyMans();
        
        FlatSigningProvider provider;
        std::string error;
        std::unique_ptr<Descriptor> desc = Parse("combo(" + EncodeSecret(test.coinbaseKey) + ")", provider, error,  false);
        assert(desc);
        WalletDescriptor w_desc(std::move(desc), 0, 0, 1, 1);
        if (!wallet->AddWalletDescriptor(w_desc, provider, "", false)) assert(false);
        CTxDestination dest = GetDestinationForKey(test.coinbaseKey.GetPubKey(), wallet->m_default_address_type);
        wallet->SetAddressBook(dest, "", "receive");
        wallet->SetLastBlockProcessed(105, WITH_LOCK(node.context()->chainman->GetMutex(), return node.context()->chainman->ActiveChain().Tip()->GetBlockHash()));
    }