### 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_
if (!pubkey) {
            error = strprintf("combo(): %s", error);
            return nullptr;
        }
        ++key_exp_index;
        return std::make_unique<ComboDescriptor>(std::move(pubkey));
    } else if (Func("combo", expr)) {
        error = "Can only have combo() at top level";
        return nullptr;
    }
    const bool multi = Func("multi", expr);
    const bool sortedmulti = !multi && Func("sortedmulti", expr);
    const bool multi_a = !(multi || sortedmulti) && Func("multi_a", expr);
    const bool sortedmulti_a = !(multi || sortedmulti || multi_a) && Func("sortedmulti_a", expr);
    if (((ctx == ParseScriptContext::TOP || ctx == ParseScriptContext::P2SH || ctx == ParseScriptContext::P2WSH) && (multi || sortedmulti)) ||
        (ctx == ParseScriptContext::P2TR && (multi_a || sortedmulti_a))) {
        auto threshold = Expr(expr);
        uint32_t thres;
        std::vector<std::unique_ptr<PubkeyProvider>> providers;
        if (!ParseUInt32(std::string(threshold.begin(), threshold.end()), &thres)) {
            error = strprintf("Multi threshold '%s' is not valid", std::string(threshold.begin(), threshold.end()));
            return nullptr;
        }
        size_t script_size = 0;
        while (expr.size()) {
            if (!Const(",", expr)) {
                error = strprintf("Multi: expected ',', got '%c'", expr[0]);
                return nullptr;
            }
            auto arg = Expr(expr);