GET /v2/transactions/pending returns empty array after a catchup

### Subject of the issue

When a node is brought up for the first time or after a period of being down and a catchup is performed, it doesn't appear to return the pending transactions after it's finished syncing. It can easily be resolved by bringing the node down and up again, but I imagine there's something preventing it from working straight away. This has been experienced in both sandbox and a local node.

### Your environment

All versions I've used over the months. Currently:

3.4.2.dev [rel/stable] (commit #54c3c397)
go-algorand is licensed with AGPLv3.0
source code available at https://github.com/algorand/go-algorand

macOS Monterey 12.2.1

### Steps to reproduce

1. `./sandbox clean`
2. `./sandbox up mainnet -v`
\*wait for catchup to complete\*
3. Check sync status `./sandbox goal node status -w 100`
\*wait until synced\*
4. `curl localhost:4001/v2/transactions/pending -H "X-Algo-API-Token: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"`
```
{
"top-transactions": [],
"total-transactions": 0
}
```
5. `./sandbox down`
6. `./sandbox up`
7. `curl localhost:4001/v2/transactions/pending -H "X-Algo-API-Token: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"`
```
{
"top-transactions": [
...txns now being returned...
],
"total-transactions": 72
}
```

### Expected behaviour

I'd expect pending transactions to be available once the node has caught up without restarting it.

### Actual behaviour

Restarting the node is required for the pending transactions to be returned.