When you need to merge the chunk index as you deleted the local chunk index/cache, then you always get this log output: (when piping/directing it to a log file as you usually do in automated systems):

Merging chunk index... (0.00%)
Merging chunk index... (1.22%)
Merging chunk index... (3.32%)
[…]
Merging chunk index... (100.00%)

(no original log file, just how I remember it)

However the command I executed there (borg create), did not have the --progress parameter passed, but just some usual ones (-v and --stats), whcih do have such a progress bar/indicator. Thus, IMHO, the behaviour of borg is wrong here. The output without --progress should be something like Merging chunk index... without any percentage.

v1.0.10