influx does not backup current shard

I have a metrics database with this retention policy
SHOW RETENTION POLICIES ON metrics
```
name duration shardGroupDuration replicaN default
---- -------- ------------------ -------- -------
autogen 1440h0m0s 24h0m0s 1 true

SHOW SHARDS
...
5555 metrics autogen 5555 2021-08-04T00:00:00Z 2021-08-05T00:00:00Z 2021-10-04T00:00:00Z
5556 metrics autogen 5556 2021-08-05T00:00:00Z 2021-08-06T00:00:00Z 2021-10-05T00:00:00Z
```
when I use
```
influxd backup -portable -host remote_host:8088 /tmp/backup
```
i have the following problem:
influx does not backup current shard
```
4.0K 20210805T122805Z.s5556.tar.gz
```
but if I use
```
influxd backup -portable -database metrics -shard 5556 -host remote_host:8088 /tmp/backup
```
Everything is ok
```
441.5M 20210805T122805Z.s5556.tar.gz
```
and when I make a backup locally everything is ok too

__Expected behavior:__
influx backup current shard

__Actual behavior:__
influx does not backup current shard

__Environment info:__
InfluxDB v1.8.0 (git: 1.8 781490de48220d7695a05c29e5a36f550a4568f5)

Reproduce the portable backup using the remote host and compare it with the explicit -database and -shard backup commands shown in the issue. Inspect the backup command's shard-selection behavior and verify that the current shard's archive contains its data; done means the default portable backup includes the current shard contents.

Written by the indexing model from the issue text.