EDIT : Originally posted as an XML issue, file format was found to be actually JSON, yet the problem is essentially the same: Import worked in 11.2 but not in 11.3. As per Hans' comments, replicated in 11.2.
This JSON file was extracted from PDF. In 11.2 import worked correctly (was able to save it as MX file). (It’s available via this Google drive link since it’s over 10MB)
Import w/ no options, with option "JSON", "RawJSON", and "ExpressionJSON" - as well as
ImportString - all failed in 11.3.
The original PDF is found here.