Hello,
i have the same problem and a solution
The keywords for the product exists at database , eg for product 23750 , exists 23750 keyword
The temporaly solution for me is to change default category for the product to another category, then change back to correct category!
Strange but it is working!
///edit Apr 17 2020 //
It only needs to add product to another category and remove it again. no need to change product default category