FCP Render defects

I use standard keyer. After export I look at video and see some corrupted areas with duration around 0.5-1.5 seconds (see screenshot below). Tried different ways to export - all gives the result at the same areas of video.

• Pre-render at FCP shows the same defects as render
• Initial footage at the same area looks just OK
• Reseting keyer parameters (e.g. slightly change any parameter) - recalulates the picture without defect.
• Changing this parameter back to initial value -> defect appears again from cache.
• Even when I get rid of defect in pre-render by changing something it comes back after export in the same place.
• Tried "Delete Generated Library Files" to clear cache - made no difference
• New exports gives corruptions at same places but.... they creating new ones in places that was OK.
• Source footage is 4k video shot on iPhone using ProMovie for manual exposure and whitebalance.
• I use external SSD
• No plugins

FCP 10.4.7 and below as well