Answer
This isn't a caching or setup mistake — it's a documented framework limitation: Custom Attributes connected to a Standard Entity aren't supported when configuring Projections (as opposed to Customizations, which have fuller custom-field support). If you need a custom field to genuinely appear across multiple unrelated standard entities' pages via Configuration, this specific combination isn't currently possible; you'd need to look at a Customization-based approach (through Developer Studio) instead if that cross-entity requirement is firm.