getblockchaininfo returns blocks > headers when update from 0.18.5 to 0.18.6

### Describe the issue
When I update bitcoin abc from `0.18.5` to `0.18.6` and run `bitcoin-cli getblockchaininfo` on testnet, it returns blocks > headers.
```
{
"chain": "test",
"blocks": 1286392,
"headers": 1286288,
"bestblockhash": "000000000000a77a83ec1dbbda5c8197358ff923467defea577230885a1e167a",
"difficulty": 1,
"mediantime": 1550287157,
"verificationprogress": 0.9999991773758619,
"chainwork": "0000000000000000000000000000000000000000000000439e9c205f379d5d51",
"pruned": false,
"softforks": [
{
"id": "bip34",
"version": 2,
"reject": {
"status": true
}
},
{
"id": "bip66",
"version": 3,
"reject": {
"status": true
}
},
{
"id": "bip65",
"version": 4,
"reject": {
"status": true
}
},
{
"id": "csv",
"version": 5,
"reject": {
"status": true
}
}
]
}
```

### Expected behaviour
blocks <= headers

### Actual behaviour
blocks > header

### Machine specs:
- OS: MacOS
- CPU: 2.9 GHz Intel Core i9
- RAM: 32Gb
- Disk size: 256Gb
- Disk Type (HD/SDD): SDD

After update, I tried run bitcoin-abc with [`-reindex`](https://bitcoin.stackexchange.com/questions/60709/when-should-i-use-reindex-chainstate-and-when-reindex/60711) option, but the blocks number is still bigger than headers number.
Result of `bitcoin-cli getnetworkinfo`
```
{
"version": 180600,
"subversion": "/Bitcoin ABC:0.18.6(EB32.0)/",
"protocolversion": 70015,
"localservices": "0000000000000025",
"localrelay": true,
"timeoffset": -1,
"networkactive": true,
"connections": 8,
"networks": [
{
"name": "ipv4",
"limited": false,
"reachable": true,
"proxy": "",
"proxy_randomize_credentials": false
},
{
"name": "ipv6",
"limited": false,
"reachable": true,
"proxy": "",
"proxy_randomize_credentials": false
},
{
"name": "onion",
"limited": true,
"reachable": false,
"proxy": "",
"proxy_randomize_credentials": false
}
],
"relayfee": 0.00001000,
"excessutxocharge": 0.00000000,
"localaddresses": [
],
"warnings": ""
}
```

Start by reproducing the testnet upgrade scenario from Bitcoin ABC 0.18.5 to 0.18.6 and inspect the implementation behind bitcoin-cli getblockchaininfo, including behavior after -reindex. Compare the reported blocks and headers values with the chain state; done means the RPC no longer reports blocks greater than headers and the behavior is verified after reindexing.

Written by the indexing model from the issue text.