Verified with steps in description in 6.6.0-7861:
[root@node1-mad-hatter-testing-centos7 bin]# ./cbbackupmgr merge -a /tmp/entbackup/ -r backup --start 2020-07-09T11_13_30.250764423-07_00 --end 2020-07-09T11_27_58.415507775-07_00
(1/6) Merging backup '2020-07-09T11_13_30.250764423-07_00'
Copied all data in 4.922372899s (Avg. 195.95KB/Sec) 10000 items / 783.81KB
[======================================================================================================================================================================================================================================================================] 100.00%
Merge bucket 'default' succeeded
Mutations merged: 10000, Mutations failed to merged: 0
Deletions merged: 0, Deletions failed to merged: 0
(2/6) Merging backup '2020-07-09T11_13_52.941198062-07_00'
Copied all data in 4.908067918s (Avg. 286.28KB/Sec) 15000 items / 1.12MB
[======================================================================================================================================================================================================================================================================] 100.00%
Merge bucket 'default' succeeded
Mutations merged: 15000, Mutations failed to merged: 0
Deletions merged: 0, Deletions failed to merged: 0
(3/6) Merging backup '2020-07-09T11_14_09.662222502-07_00'
Copied all data in 5.059106021s (Avg. 301.29KB/Sec) 20000 items / 1.47MB
[======================================================================================================================================================================================================================================================================] 100.00%
Merge bucket 'default' succeeded
Mutations merged: 20000, Mutations failed to merged: 0
Deletions merged: 0, Deletions failed to merged: 0
(4/6) Merging backup '2020-07-09T11_22_45.949655018-07_00'
Copied all data in 8.025484135s (Avg. 218.21KB/Sec) 25000 items / 1.70MB
[======================================================================================================================================================================================================================================================================] 100.00%
Merge bucket 'default' succeeded
Mutations merged: 25000, Mutations failed to merged: 0
Deletions merged: 0, Deletions failed to merged: 0
(5/6) Merging backup '2020-07-09T11_27_10.003238822-07_00'
Copied all data in 8.545139194s (Avg. 260.33KB/Sec) 30000 items / 2.03MB
[======================================================================================================================================================================================================================================================================] 100.00%
Merge bucket 'default' succeeded
Mutations merged: 30000, Mutations failed to merged: 0
Deletions merged: 0, Deletions failed to merged: 0
(6/6) Merging backup '2020-07-09T11_27_58.415507775-07_00'
Copied all data in 8.492812354s (Avg. 302.44KB/Sec) 35000 items / 2.36MB
[======================================================================================================================================================================================================================================================================] 100.00%
Merge bucket 'default' succeeded
Mutations merged: 35000, Mutations failed to merged: 0
Deletions merged: 0, Deletions failed to merged: 0
Merge completed successfully
I will verify it when the build with this fixes is ready