Merging backups results in unexpected number of mutations in docs

Some of our merge tests are failing because of an unexpected number of mutations in a doc (the test is expecting 1 mutation but get 3). The rest of the parameters are as expected. For example:

{'test_docs-89': {'expected': ['{"age": 4, "first_name": "sharon", "_id": "test_docs-89", "mutated": 1}'], 'actual': ['{"age": 4, "first_name": "sharon", "_id": "test_docs-89", "mutated": 3}']} 

The steps that the test is doing are:

1) Create 5 backups

2) Merge the backups

3) Restore them.

I have attached the Jenkins logs, the cbbackmgr calls (extracted from the Jenkins logs) and cbbackupmgr collect-logs archive.

Build: 7.2.1-5904

CC: James Lee