### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current behaviour

I randomly encountered this by loading a wallet created from master with an older version. I could trace it down to bd13dc2f46ea10302a928fcf0f53b7aed77ad260 (#26076), but I don't know descriptor wallets enough to understand what's going on (cc @Sjors @achow101).

### Expected behaviour

`bitcoind` starts, or at least tells me to use a later version if #26076 was a breaking change instead of corrupting `wallet.dat`.

### Steps to reproduce

(master, clean chain): 
`createwallet -regtest ""`
(v25.0rc2):
first `bitcoind -regtest` succeeds
then stop and restart the node: The second `bitcoind -regtest` leads to an InitError:
`Error: Error: Unable to expand wallet descriptor from cache`
now, the wallet cannot be loaded by either version.

### Relevant log output

_No response_

### How did you obtain Bitcoin Core

Compiled from source

### What version of Bitcoin Core are you using?

v25.0rc2 / master

### Operating system and version

Ubuntu

### Machine specifications

_No response_
{
    LOCK(cs_desc_man);
    
    if (m_wallet_descriptor.next_index - 1 == index) {
        m_wallet_descriptor.next_index--;
    }
    WalletBatch(m_storage.GetDatabase()).WriteDescriptor(GetID(), m_wallet_descriptor);
    NotifyCanGetAddressesChanged();
}
std::map<CKeyID, CKey> DescriptorScriptPubKeyMan::GetKeys() const
{
    AssertLockHeld(cs_desc_man);
    if (m_storage.HasEncryptionKeys() && !m_storage.IsLocked()) {
        KeyMap keys;
        for (const auto& key_pair : m_map_crypted_keys) {
            const CPubKey& pubkey = key_pair.second.first;
            const std::vector<unsigned char>& crypted_secret = key_pair.second.second;
            CKey key;
            DecryptKey(m_storage.GetEncryptionKey(), crypted_secret, pubkey, key);
            keys[pubkey.GetID()] = key;
        }
        return keys;
    }
    return m_map_keys;
}
bool DescriptorScriptPubKeyMan::TopUp(unsigned int size)