### 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_
{
private:
    miniscript::NodeRef<uint32_t> m_node;
protected:
    std::vector<CScript> MakeScripts(const std::vector<CPubKey>& keys, Span<const CScript> scripts,
                                     FlatSigningProvider& provider) const override
    {
        for (const auto& key : keys) provider.pubkeys.emplace(key.GetID(), key);
        return Vector(m_node->ToScript(ScriptMaker(keys)));
    }
public:
    MiniscriptDescriptor(std::vector<std::unique_ptr<PubkeyProvider>> providers, miniscript::NodeRef<uint32_t> node)
        : DescriptorImpl(std::move(providers), "?"), m_node(std::move(node)) {}
    bool ToStringHelper(const SigningProvider* arg, std::string& out, const StringType type,
                        const DescriptorCache* cache = nullptr) const override
    {
        if (const auto res = m_node->ToString(StringMaker(arg, m_pubkey_args, type == StringType::PRIVATE))) {
            out = *res;
            return true;
        }
        return false;
    }
    bool IsSolvable() const override { return true; }
    bool IsSingleType() const final { return true; }
};
class RawTRDescriptor final : public DescriptorImpl
{
protected:
    std::vector<CScript> MakeScripts(const std::vector<CPubKey>& keys, Span<const CScript> scripts, FlatSigningProvider& out) const override