I was testing with different format strings for the Tag->Filename conversion. The program started behaving unexpectedly when the string was getting rather long - I could type more characters, but every keypress took me to the beginning of the string in addition to adding the character I wanted.
In my string this happened with exactly 261 characters, but with a continuous character string without white space or special characters the limit was at 255.
Thanks for the swift bug fix! It works now from the selection dialog, but the same behavior seems to exist when editing the line from File->Options->Convert.