Try this:

you get:
-
* /rename: unable to rename '#mircdir#\TEST_W~1.TXT' to '#mircdir#\Test_with_long_filename.txt'
-
(i try to rename the filename, i change le first "t" in "T" (upper)
mIRC can rename test_with_long_filename.txt to Test_with_long_filename.txt but not here
mIRC can rename $shortfn(test_with_long_filename.txt) to a $longfn(Test_with_long_filename.txt) but not here, because test_with_long_filename.txt = Test_with_long_filename.txt, but test_with_long_filename.txt !=== Test_with_long_filename.txt

... understanding ??!!

It's a long time 'bug'. mIRC simply cannot rename to a file that already exists.