On 6 different Routerboard including RB450/RB2011/RB912-2n/RB912-UiHnD/.. (some of them netinstalled to be sure), changed 3 different PC and testing with 3 different browser (Chrome,Firefox,IE) clearing cache every time.
Downgrading to 6.20 (or 6.19) solve immediately the problem, upgrading to newer 6.23/6.24/6.25rc[45] problem is back.
Re-tested now on a RB912UAG-2HPnD (netinstalled 6.24 and, after test, also upgraded to 6.25rc5), problem is there.
Re-tested now also without custom branding (a clean delete/set netinstall 6.24 on RB912UAGHPnD) and hotspot user page shows hidden fields (see image above).
Mind you.. almost all sections are ok (never checked all obviously), the hotspot user managing one is surely not ok IMHO.