Alter MirrorSourceConnector offsets dont work

In connect-offsets topic:

the offsets wrote by connector, key is `{"cluster":"A","partition":2,"topic":"topic"}`

after alter offsets, the key is  `{"partition":2,"topic":"topic","cluster":"A"}`

in Worker.globalOffsetBackingStore.data, both two keys exist, because the are different strings:

{"cluster":"A","partition":2,"topic":"topic"} {"offset":2}

 

{"partition":2,"topic":"topic","cluster":"A"}{"offset":3}

So alter offsets is not succussful, because when get offsets from globalOffsetBackingStore, always returns 

{"offset":2}