The same samples do not reproduce the problem in After Effects on Windows 10. I do not reproduce this problem with any other H.264 footage, just the Canon 5D samples that I have. In more concrete terms: a 1920x1080 frame renders vertically scaled (squished) to 1920x952, with the last 128 rows filled with black. The bottom 128 pixels of the footage are black, and the footage is vertically scaled so that it fits within the cropped dimensions. mov footage when hardware-accelerated H.264 decoding is enabled on Mac. It appears that After Effects 16.0 (and Premiere Pro 13.0) are mis-sizing Canon 5D H.264.

Restart After Effects, then purge the cache via Edit > Purge > All Memory & Disk Cache. Disable the 'Enable hardware accelerated decoding' option in Preferences > Import. The Headless Leader at Adobe strikes again. This place is a huge resource is a godsend to folks like me I'm way out on the bright side of nowhere here. In AME, I created an encoding Preset where I didn't select the 'Match Source' tick box and instead set the Height & Width dimensions manually to 1920x1080.

Davinci resolve support h.264