One possibility could be that the head switching point is very off. You can try to look for a pot/trimmer, typically labeled PG shifter on the panasonics, and see if it helps to adjust it. It determines where in the picture the vcr switches between the two active video heads on the video head drum. Note the original position before adjusting it though! Adjustment can be very sensitive.
It could be something completely different though, I'm not an expert by any means, but I had something similar happen when tweaking the head switching point it on a panasonic VCR we have.
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