Honestly have no clue what this one could be. Some kind of teaser demo?
Basically all of Team Maniax (SMT, Etrian Odyssey and TMS) have been on SMT5.
I forget, but when Atlus puts out a Vanillaware game, how does it show up on rating boards? Because there was an unnamed fantasy teaser as part of the Japan exclusive paid demo for 13 Sentinels:
The “Extras” menu in the newly released 13 Sentinels: Aegis Rim Prologue includes a trailer labeled “????” that teases an unannounced, Atlus-published VanillaWare project.
www.gematsu.com
That was over 2 years ago now.
EDIT: They were hiring for that game in 2017, so it would line up as something to be officially announced soon.
Pen could refer to it being some kind of pen and paper inspired thing.
EDIT 2: Other Vanillaware games from Atlus cite Vanillaware as the author, so this is from an internal team.