Replica is not syncronized

Hey,
I just had a very strange problem in my prod environment.
we have 2 servers one contains replicas of all the indices of the second.
I just saw that one of the indices replicas (not in the master) stopped
synchronizing with the master and was out of date for the past few days.
other indices continued to be updated successfully.
once I stopped that node deleted the index and restarted it, the replica
created successfully with all the data from the master and the
synchronization was OK.
I think that the problem may be connected to a backup process that I have
that first disable flushing for an index copy the index directory and
enable the flushing again. could it be?
although that now its OK I'm afraid that the problem will return.
anyone experienced a similar issue?
what can it be?

Thanks.

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