Answer
These are two genuinely different standard views, not two names for the same thing — the _pub-suffixed view and the plain view are connected to two separate standard views (commonly used by two different IFS screens for their own distinct purposes), so custom fields added to one don't automatically appear on the other. Alternate view suffixes like _UIV, _PUB, _BI, _LOV, and _LIST aren't automatically or mechanically generated variants of a base view at all — they're deliberately created by IFS's own developers for specific distinct purposes. Custom fields need to be added separately to whichever specific view(s) the screen(s) you actually care about are genuinely built on, rather than assuming a _pub-suffixed view will automatically inherit fields added to its plain counterpart.