We're a little bit inconsistent with this as we handle it in some places but not in others but for the most part this is not totally unexpected.
You are not passing a boolean value of false
to the template tag here, you are passing a string value of 'false'
which is actually a truthy value because it's not an empty string.
The correct way to set this to false is to pass an actual boolean value which would be done with showtitle="{{ false }}"
.
There should probably be a wider project in order to make this more consistent but not dealing with this particular case as a bug right now.