Yep, fixed it for me, TV was unwatchable with interlacing enabled - juddering and out of sync mess. Rpi4 + TVHeadend. Switched to a totally different system (Xbian) because it was driving me nuts. Didn't make any sense at all that it could play a 160GB 4k video but not 1080i/480i TV. Tried three different PVR backends/frontend over the last few days!
Worth remembering that h.264 and interlaced content are handled by totally separate parts of the Pi4B SoC to h.265/hevc. The h.264/interlaced stuff is decoded and deinterlaced by the legacy VPU that remains the same as the Pi 3B+ and before, whereas the hevc/h.265 decoder is a separate unit and handled separately. I believe some builds at the moment are not handling stuff particularly effectively using the old decoder?