Whoever built the wall did not prepare a proper base. You have no way to tell what will happen in the future.
You do not have to tear down the entire wall but do a stair step down to the sunken area of the base material. Dig out the soft base and replace with a compacted base that is level and then rebuild the wall with units you reomved. - They will match.
A common problem with a lack of proper preparation that comes with all retaining wall types. It is good it was not a poured or rigid wall that had a poor base and footing.
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