Will leaving the implant capsule (under muscle) interfere with mammograms?

Because the capsule is behind the muscle it is not likely to interfere with mammograms. Your surgeon should however make sure that the muscle is re-attached, which will also help the capsule to close up and heal.

A normal capsule is very thin and should not interfere with mammogram.  Also the Mammogram focuses on the breast tissue above the muscle not what is underneath the muscle.  If there is any concern in your breast screening down the line, Ultrasound, 3D mammogram or MRI are useful additional studies.

Just like the presence of a breast implant shouldn't interfere with a mammogram and the correct diagnosis of breast cancer, the capsule should not interfere with a mammogram.

If the capsule was thin, it will likely resorb and likely should not impact your mammograms to any significant extent.

With under the muscle implants, the capsules are behind the breast tissue and muscle, so they won't get in the way of mammograms or breast exams.  As long as they are not thick and calcified, they are expected to resolve on their own after implant removal.