This is a normal part of how YT validates views. Temporarily slowing down/freezing a video's view count is expected, here's more info: https://goo.gl/S7DIry . Also, we regularly verify the legitimacy of the accounts that are subscribed to your channel: https://goo.gl/QGgyKe
We'd like to know more. Could you check with your subs, if is this happening for desktop, mobile or email notifications (or all of the above)? Also, is this happening with *all* notifications or just certain types? Appreciate the additional info!