NULL is generated automatically. There used to be no issue in V1 import. I can replace all NULL with empty without issue. But these should not be the reason for now inputting to category correctly. post_category in DB showing cat_ID is inserted and correct.
Default category is one of the category in that CPT, “Others”, which is to be replaced with uncategorized, sound better.
So form the screen shot, did you discover any possible reason for listing not showing in category?
cheers