Coldfusion 9 and 10, and Railo 4.2 are all happy with the following, as is the decoder at https://www.base64decode.org/;
Any number of = can be place at the end and they still happily decode the string.
Lucee is throwing error 'can't decode the base64 input string [...], because the input string has an invalid length'. It will work only if you remove the '='. Obviously this is a simple fix so not a serious bug but could affect inter-system operability