While playing in PIE, the editor switches back to the main tab when you're in the blueprint tab after you've pressed 'stop'. Therefore, if you're debugging a situation within the blueprint, you're interrupted due to the tab switch.
Also tested in 4.8: UE4/Promoted-CL-2498147
1. Open FPS template
2. Press 'play' to PIE
3. Open the level bp
4. Press 'stop' in the level bp
Head over to the existing Questions & Answers thread and let us know what's up.