Content being rendered as empty string in posts endpoint

My guess is, there is a filter causing change to the post content on api call. Could you check if the problem persists only in api or subscriber can see the content on normal page?

The Restrict Content by Role plugin might be setting some rules that disallow subscriber role to view content in api calls.

I would start with disabling/enabling that plugin to see if that’s the cause. If not, I would do the same with other plugins one by one to verify if problem is caused by particular plugin.

If the problem persists outside of installed plugins, there must be custom code (custom theme or child theme), filtering post contents. If that’s the case I recommend looking through the list of filters to find ones that might be worth hooking into. If problem is in api only I would start with hooks prefixed with rest_.