formatDateTime problem with seconds

I'm having a problem which I've been staring at for about an hour or so. I can't explain what is happening here. I use this expression

formatDateTime([%CurrentDateTime%], 'yyyy-MM-dd''T''HH:mm:Z')

which results in:

2017-09-05T16:57:+0200

Then I tried this one

formatDateTime([%CurrentDateTime%], 'yyyy-MM-dd''T''HH:mm:sZ')

which results in:

2017-09-05T17:22:9+0200

Last one I tried (and the one I actually need :) ) is:

formatDateTime([%CurrentDateTime%], 'yyyy-MM-dd''T''HH:mm:ssZ')

which results in this:

09/05/2017 17:24:39

And I can't explain the difference. Except that in the second case the format might be invalid and it reverts to a default. But I don't think the format is invalid, so I'm guessing a bug?