It's unforunately one of the issues with the M&P. I've personally broken a striker not long after getting my first M&P. Since that time I've used quality snap caps like the A-Zoom and I haven't experienced any further breakages. S&W used a redesigned striker after a while to minimize the occurrence of broken strikers.

My advice to you would be to use the bypass lever to take the weapon down for cleaning, and to use snapcaps when doing dryfire training. Another good option is to buy a spare striker assembly from S&W and keep it as a dedicated dryfire striker. It wouldn't hurt to keep a spare striker on hand anyway.