Our ISP doesn't force MITM certificates, but used to try to mess with the retrieved pages sporadically with stream-hijacks (you navigate to HN, and got greeted with a full page ad) or banner insertions, forcing connection to mixed status while keeping the inner frame HTTPS.

They're not doing this anymore, because I guess they now know how to use their DPI infra in useful ways to them.

My mobile carrier still injects stuff to HTTP pages, but doesn't mess with HTTPS ones, at least yet.