base64 command on macOS returns wrong result

Decoding dXNlcm5hbWU6cGFzc3dvcmQK gives username:password\n.

So my guess is that in both cases, you have a linefeed you weren't expecting. The testin.txt case is easy to solve (modify file, remove linefeed).

I expect the base64 case can be solved by using echo with the -n parameter to remove the linefeed:

echo -n username:password | base64 --encode