"Acanthopholis" class does not appear in this NPR's ship list, despite there being dozens of them still remaining. (There are none at the bottom of the list scrolled down either, I checked.) I boarded a few of their Acanthopholis class, so perhaps this made it disappear when the captured ships were transferred to me?
Ideally, the ship class should not disappear from the NPR intel ship list if I capture one. However, the presumed ship type/hull and full details
should be revealed to me upon capture (so it doesn't show as XX). At the bare minimum, if it remained on the ship list then I could set the ship hull manually myself.
Edit: It turns out this ship design actually
is on the alien intel list, but it is now listed under its NPR-assigned class name "XX Southampton". I do not have "Use Real Ship / Class Names" selected, so this behavior was unexpected and caused the disconnect that we see in the game--already discovered ships of this class (seen on the System Map) keep their "Acanthopholis" name, while on the Alien Intel screen they have been renamed to the Real Ship / Class Name. The class should ideally not have been renamed in my intel screen.