Don't ask me how I found this one, because I'm not even sure myself. Here are the steps to reproduce it.
1) Create a blank document. 2) Add a Heading component. 3) Add a second Heading component directly underneath.
Your page should now just look like a big, white space with the words Heading Heading
4) Click the bottom heading component and assign it a class via the Attributes panel > Class Names field. (It could be any BS class, but for the purposes of this exercise, let's make it ml-5) 5) Next select the top heading component (you will notice the bottom word Heading moves to the right as the ml-5 class is applied.) 6) With the top heading component selected, assign it a class (let's use text-primary) and then immediately click your mouse in the ID field in the Attributes panel. 7) Now just click the bottom heading component.
You should see top heading component's class change to ml-5, which is what we assigned to the bottom component. Somehow, having the cursor in the ID field when clicking on components causes screwy things to happen with assigned classes.