Is this specific to certain types of MP4 footage files, or all of them?
Did it start with a recent 17.1 build, or is this the first build of it you installed?
Is this same footage displaying correctly in the current Beta version of PPro?
The most recent beta updates seem to have fixed the issue. I had MP4 files of stock footage from Videoblocks from a few years back. Didn't run the test of this in Premiere Pro though.