How do your tags looks like in the EAC window, do they contain any unicode characters? Maybe the problem is that EAC add tags in console (mpcenc is a console application, anyway) so maybe the unicode detection does not work well?
You can try to add --unicode to your command line (Compression Options -> External Compression -> Additional command-line options):
Code:
--unicode Toggle unicode input from console on/off if autodetection
doesn't work. This switch MUST be before any tag switch.