This is a bit of a specific bug.
I'm noticing that if there's a Next Frame Event action in an FSM, and the PlayMaker editor is open, then when you hit Play, you can no longer click in any numerical field in the Inspector to edit it.
For instance, in Play mode, you can't click in the Transform component to type in new x, y, or z values. However, you can still click on the "X" label and drag it around and make changes that way, but this lacks the precision of simply typing a value into the float field.
When you exit Play mode, you can then click and type new values as usual. In addition, if you're in Play mode and close the PlayMaker editor, or remove the Next Frame Event, you can click and type in the fields again.
This is in Unity 2018.2.20f1 (our artist notices it in 13f1 as well). I have an empty project (with PM installed and a test scene) which reproduces the problem (~22MB).