Interesting problem. The cause appears to be Unicode characters in your file being converted to high ANSI characters. MSFT has changed the behavior of an underlying Win32 function used by the Map related functions to add new key/value pairs to a map and that is tripping up the map functions when processing high ANSI characters. It will be corrected in the next release of WinBatch. Until then you could convert your script to using the COM Automation dictionary object, "scripting.dictionary". It isn't quite as fast as WIL Maps but it might serve your needs. You could also just use regular WIL arrays to accomplish the same task with a little extra processing.
Thanks for taking the time to report the problem and also for providing a reproducing example. Really appreciate your efforts.