I have a Magento 1.14.2.` store with about 1500 products in about 50 categories and sub-categories.
A small number of products (2 dozen?) in a few particular categories regularly "disappear" every few minutes from a handful of categories, but not the other categories that they may be assigned to. Other products in the affected categories appear at all times.
The missing products reappear when I manually re-index and clear the FPC manually.
These are "old" products and categories that I've had on the site for a few years (without any known issues) and not newly imported or anything. I've checked the visibility/status settings for the products, etc. and the category settings, etc. but don't see anything unusual. Also, these few products seem to behave exactly as expected when assigned to other categories.
I've tried clearing the category assignments for these affected products and reassigning them--that seems to have worked in the past when something like this was going on--but not this time. I've recreated some of the affected categories, but that doesn't seem to help either.
Does anyone have an idea what might be causing this or how to correct it?