At some point during development unknown; the category select field has gone missing from the product edit screen (on all product types).
It's possible it happened during a partial update of content from M1 to M2 as we brought a new development site up to date with it's original M1 data.
Is it possible that the category field is not being output due to an error pulling in categories? The category tree and site catalog as a whole work fine so this wasn't picked up till much later on.
The main thing I'm struggling with here is how to debug the product edit screen.
I'm reviewing the edit screen with template hints on. I can see that the fields look like they are output by js in the templates/form/collapsible.xhtml file.
What I don't know how to do is find and debug the php that's building the data for the fields.
What would be the best way to debug this problem (not php debugging in general but specific to the Magento admin)?
There are no errors in the console. And the exception, system and debug logs of Magento only show the usual cache and cron notifications.