If this is for the Expedition flow then HETERO isn't used, the fractures are managed by the parts data (PDB) in Library Manager. If it is for netlist flow then Part Number should be DEVICE and Part Name isn't used. Also for the Expedition flow the U? in Ref Designator is unnecessary, the assignment is made from the Reference Des Prefix setting in the first page of the Part Editor.
If you are sure you need to add the HETERO property thej check how many characters are allowed for the property in the Property Definition Editor, you may have reached the maximum set here, the setting has a maximum value of 255.
Filenames are converted to lower case to support cross-platform support on Linux and Unix. The different case of the symbols should not affect the tools but if you wish to have them all in the same case you could try the following, though I'd recommend you do this on a copy of the library (assuming you are using a Central Library). Rename the symbol files to the case you require via the Operating System (file manager etc). Rename the sysindex.cbf file to sysindex,bak and open the library. Library Manager will re-index the library with the new file names (you may not see a difference in all areas of the tool after this).
If using the netlist flow simply rename the files.