I was recently informed by Fanbox that the last work I posted (La Signora AR) is considered R-18, and was advised to set this Fanbox account to an R-18 account.
I'm not entirely sure why that last one flagged their system and not the earlier ones, but in order to avoid any trouble, I've set this account to R-18.
Unlike Patreon or deviantArt, it doesn't seem like they have a problem with my content itself - just the fact that I didn't set it as R-18.
I heard that you can't pledge to an R-18 Fanbox account using Paypal, but I'm not sure whether that's true. We'll soon find out I guess.