One of the options for "readtable" is "ReadRowNames".
Apparently, when "ReadRowNames" is true, Matlab silently truncates the length of rownames to a maximum of 63 characters.
However, if you manually assigned row names to a table in Matlab, they can be of arbitrary length. (e.g. myTable.Properties.RowNames = cellArrayOfVeryLongStrings works and does not truncate row names.)
No warning is given, and this "feature" is not mentioned in any documentation or Matlab tutorial on Tables.
Note that Matlab issues various warnings when, say, Variable Names for table are too long, non-unique, contain invalid characters, etc. The crime here is that Matlab automatically "formats" incoming rownames in "readtable" without any warning or notification.