I am currently trying to disable or tweak the new "headline-style" that was introduced with TM2 (or the bundle?).
The undesired behaviour will change the font-size and family for the line depending on the headline level.
Here is an example output:

I already tried to change themes and inspected the bundle sourcecode but i can't find anything related to this.
How can i disable/remove this?
In the Bundles menu, select Edit Bundles….
Then select the bundle Themes and open the Settings.
There you can disable each Markup: Heading #, change the sizes or even delete them. (Remember to close and reopen your document for changes to apply)