labeldrop doesn't seem to drop all matching labels

I have the following simple relabel config:
```json
[
{
"source_labels": ["__aws_cloudwatch_log_group"],
"target_label": "cloudwatch_log_group",
"action": "replace"
},
{
"regex": "__aws_.*",
"action": "labeldrop"
}
]
```
Labels before applying the config:
```
__aws_cloudwatch_log_group: /aws/transfer/s-7a8a3980081f4479b
__aws_cloudwatch_owner: XXXXXXXXXXXX
__aws_kinesis_event_source_arn: arn:aws:kinesis:ap-southeast-2:XXXXXXXXXXXX:stream/cloudwatch.logs
__aws_log_type: kinesis
```
Labels after applying the config:
```
__aws_cloudwatch_log_group: /aws/transfer/s-7a8a3980081f4479b
__aws_cloudwatch_owner: XXXXXXXXXXXX
__aws_kinesis_event_source_arn: arn:aws:kinesis:ap-southeast-2:XXXXXXXXXXXX:stream/cloudwatch.logs
cloudwatch_log_group: /aws/transfer/s-7a8a3980081f4479b
```
So the label was replaced correctly, but then only one out of 4 matching labels was dropped.