Display Issue with Bonus Action Attacks
I've created an action on a creature and when set to use 1 Action it displays as so.
Changing the ability to cost 1 Bonus Action causes it to display like so.

This is specific to attacks that are changed to bonus actions, not features. I'll try to take a look at this this afternoon.
Previously MonsterBlock hasn't supported Attacks as anything other than an Action. This is why the the formatting isn't appearing as intended. However, with the new publications, there are attacks which are appearing in both Bonus Actions and Reactions (usually in the form of spell attacks). I'm going to change this from a bug to an enhancement.