I think we've been gaslit on the cubes by faggots from prydwen and co for a long time now. This mostly affects PvP but it's still kind of bullshit, can't help but think this kind of info was kept secret or something. Although I don't know if at some point the some of the cubes got their description changed, I don't remember the Healing Cube's desc being "HP granted" on release.
I tested the Healing Cube with Rapunzel alone to finally see if the HP Potency buff it gives was bugged like they claimed on release, or rather the anons back then talked bullshit about how it only affects the wielder getting healed and doesn't work like expected. Saw first how much she healed with the Charge Speed cube which is lvl8 then tested with the lvl5 Healing cube; she healed herself more with it, then I put Bay alongside her and both got healed by the same amount. Just now I did the same thing with Mana to make it easier to conclude since Rapu has a healing buff that happens shortly after the fight begins and the same thing happened, only this time I tested the healed nikkers with the Healing Cube equipped and no Cube, turns out the Healing Cube increase outgoing healing but not incoming healing, probably to avoid it stacking upon itself by just being equipped on the whole team. It's also worth noting that the heals were bigger than with my lvl8 Vigor (HP Up) Cube.
Now if only there was info on the Quantum Cube affecting Burst Filling skills like Siren's, Rosanna's and Helm's. Because if it turns out it does then I'm gonna be pissed about it too.