No idea about what's causing the delay for biometrics notices to be mailed out. I did notice the I-751 entry missing in the most recent CSC processing time update in the uscis.gov. I emailed the webmaster about it, no replies as of this writing yet. But I'm not expecting any reply either, hopeful but not expecting. To me, it looks like a clerical error only.
I received my biometrics letter and on the same day, went to my local ASC to have my biometrics taken on June 20 (see my siggy).
Hope you receive yours soon! Keep us posted.