The vc4 driver doesn't handle interlaced modes correctly. When an interlaced video mode is set in config.txt, as soon as the vc4 module is loaded, output switches to progressive.

To reproduce the bug add the following to config.txt:
hdmi_group=1
hdmi_mode=20
hdmi_drive=2
hdmi_pixel_encoding=3
dtoverlay=vc4-fkms-v3d

reboot and watch the output of the HDMI port. The Pi starts with 1080i and as soon as the vc4 kernel module loads, the output changes to 1080p. The same thing happens with mode 5, 6, 7, 21 and 22 but changing to 480p, 576p or 1080p respectively.

Especially when driving a CRT not supporting 1080p or only SD, this causes it to display garbage or nothing at all (and in the worst case it ends with a puff of smoke and a broken horizontal deflection driver).

This was tested on an Raspberry Pi 3B+ and 4B. Both behave the same.
Image used is release 2020-05-27
Kernel version 4.19.118-v7+ #1311