If an image field is always returning the image ID rather than another value as set in the field setting then this is a symptom of a missing or incorrect field_key in the database, or of something that is interfering with ACF formatting the value as you want. The same thing can happen with any field type that is more than a simple text value.
The first step should be to disable other plugins and switch themes to figure out figure out where the problem might be.
If you are inserting the images in some way other than the post edit or other edit page then check that the field keys are being inserted correctly into the database as well.
The fix listed on the link you sent will work if you can’t fix the cause of the issue.
