it could be that the iRS is having issues with its fax system.
or it could just be that the IRS computers are resending the notice because they are sp backed up its taking it months to get to your response and process it. until then maybe nothing is being entered in it s system that the requested documents have been received. even though you e-filed there could be something in your return that requires manual processing. only by calling the iRS could you possibly get an answer but getting through on its phone lines is also an issue
so you have two other choices
contact IRS Tax Advocate in your state
https://www.taxpayeradvocate.irs.gov/contact-us/#FindlocalTAS
or use form 911
https://www.irs.gov/pub/irs-pdf/f911.pdf
or mail the forms to the IRS at the address indicated - use a method where you get proof of delivery
