We'd like to help. Could you check with your subs, if this is 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!
Thanks for confirming on this. You can have your subs check their notification settings for the device here: https://yt.be/help/vEdZ . Also, do make sure that you select 'Notify subscribers' while publishing the video.