I was able to reproduce it - it occurs when you've got far enough in the game that the snake starts planting red saucers.
It's a bug from the original game. There is a table of screen addresses and when it plants a saucer (at the bottom of the screen) it reads beyond the end of the table and ends up incorrectly writing to zero page locations. Once the saucer is bouncing around the screen it is fine, it is only the initial placement of it where the issue occurs.
The 5200 and A8 use zero page differently which is why the bug is more evident here, although it could still lead to problems on the A8 original.
I made the table of screen addresses longer and it now seems fine, as far as I can get in the game anyway.